Add experimental "dont_push" push action to suppress push for notifications

This is a potential solution to https://github.com/vector-im/riot-web/issues/3374
and https://github.com/vector-im/riot-web/issues/5953
as raised by Mozilla at https://github.com/vector-im/riot-web/issues/10868.

This lets you define a push rule action which increases the badge count (unread notification)
count on a given room, but doesn't actually send a push for that notification via email or HTTP.
We might want to define this as the default behaviour for group chats in future
to solve https://github.com/vector-im/riot-web/issues/3268 at last.

This is implemented as a string action rather than a tweak because:
 * Other pushers don't care about the tweak, given they won't ever get pushed
 * The DB can store the tweak more efficiently using the existing `notify` table.
 * It avoids breaking the default_notif/highlight_action optimisations.

Clients which generate their own notifs (e.g. desktop notifs from Riot/Web
would need to be aware of the new push action) to uphold it.

An alternative way to do this would be to maintain a `msg_count` alongside
`highlight_count` and `notification_count` in `unread_notifications` in sync responses.
However, doing this by counting the rows in `events` since the `stream_position`
of the user's last read receipt turns out to be painfully slow (~200ms), perhaps
due to the size of the events table.  So instead, we use the highly optimised
existing event_push_actions (and event_push_actions_staging) table to maintain
the counts - using the code paths which already exist for tracking unread
notification counts efficiently.  These queries are typically ~3ms or so.

The biggest issues I see here are:
 * We're slightly repurposing the `notif` field on `event_push_actions` to
   track whether a given action actually sent a `push` or not.  This doesn't
   seem unreasonable, but it's slightly naughty given that previously the
   field explicitly tracked whether `notify` was true for the action (and
   as a result, it was uselessly always set to 1 in the DB).
 * We're going to put more load on the `event_push_actions` table for all the
   random group chats which people had previously muted. In practice i don't
   think there are many of these though.
 * There isn't an MSC for this yet (although this comment could become one).

synapse/storage/event_push_actions.py

@@ -124,8 +124,8 @@ class EventPushActionsWorkerStore(SQLBaseStore):
     def _get_unread_counts_by_pos_txn(self, txn, room_id, user_id, stream_ordering):
 
         # First get number of notifications.
-        # We don't need to put a notif=1 clause as all rows always have
+        # We ignore the notif column, given we want unread counts irrespective of
-        # notif=1
+        # whether the notification actually sent a push or not.
         sql = (
             "SELECT count(*)"
             " FROM event_push_actions ea"
@@ -223,6 +223,7 @@ class EventPushActionsWorkerStore(SQLBaseStore):
                 "   AND ep.user_id = ?"
                 "   AND ep.stream_ordering > ?"
                 "   AND ep.stream_ordering <= ?"
+                "   AND ep.notif = 1"
                 " ORDER BY ep.stream_ordering ASC LIMIT ?"
             )
             args = [user_id, user_id, min_stream_ordering, max_stream_ordering, limit]
@@ -251,6 +252,7 @@ class EventPushActionsWorkerStore(SQLBaseStore):
                 "   AND ep.user_id = ?"
                 "   AND ep.stream_ordering > ?"
                 "   AND ep.stream_ordering <= ?"
+                "   AND ep.notif = 1"
                 " ORDER BY ep.stream_ordering ASC LIMIT ?"
             )
             args = [user_id, user_id, min_stream_ordering, max_stream_ordering, limit]
@@ -323,6 +325,7 @@ class EventPushActionsWorkerStore(SQLBaseStore):
                 "   AND ep.user_id = ?"
                 "   AND ep.stream_ordering > ?"
                 "   AND ep.stream_ordering <= ?"
+                "   AND ep.notif = 1"
                 " ORDER BY ep.stream_ordering DESC LIMIT ?"
             )
             args = [user_id, user_id, min_stream_ordering, max_stream_ordering, limit]
@@ -351,6 +354,7 @@ class EventPushActionsWorkerStore(SQLBaseStore):
                 "   AND ep.user_id = ?"
                 "   AND ep.stream_ordering > ?"
                 "   AND ep.stream_ordering <= ?"
+                "   AND ep.notif = 1"
                 " ORDER BY ep.stream_ordering DESC LIMIT ?"
             )
             args = [user_id, user_id, min_stream_ordering, max_stream_ordering, limit]
@@ -400,7 +404,7 @@ class EventPushActionsWorkerStore(SQLBaseStore):
         def _get_if_maybe_push_in_range_for_user_txn(txn):
             sql = """
                 SELECT 1 FROM event_push_actions
-                WHERE user_id = ? AND stream_ordering > ?
+                WHERE user_id = ? AND stream_ordering > ? AND notif = 1
                 LIMIT 1
             """
 
@@ -429,14 +433,15 @@ class EventPushActionsWorkerStore(SQLBaseStore):
             return
 
         # This is a helper function for generating the necessary tuple that
-        # can be used to inert into the `event_push_actions_staging` table.
+        # can be used to insert into the `event_push_actions_staging` table.
         def _gen_entry(user_id, actions):
             is_highlight = 1 if _action_has_highlight(actions) else 0
+            notif = 0 if "dont_push" in actions else 1
             return (
                 event_id,  # event_id column
                 user_id,  # user_id column
                 _serialize_action(actions, is_highlight),  # actions column
-                1,  # notif column
+                notif,  # notif column
                 is_highlight,  # highlight column
             )