add default_power_level_content_override config option. (#12618)

Co-authored-by: Matthew Hodgson <matthew@matrix.org>

docs/sample_config.yaml

@@ -2468,6 +2468,40 @@ push:
 #
 #encryption_enabled_by_default_for_room_type: invite
 
+# Override the default power levels for rooms created on this server, per
+# room creation preset.
+#
+# The appropriate dictionary for the room preset will be applied on top
+# of the existing power levels content.
+#
+# Useful if you know that your users need special permissions in rooms
+# that they create (e.g. to send particular types of state events without
+# needing an elevated power level).  This takes the same shape as the
+# `power_level_content_override` parameter in the /createRoom API, but
+# is applied before that parameter.
+#
+# Valid keys are some or all of `private_chat`, `trusted_private_chat`
+# and `public_chat`. Inside each of those should be any of the
+# properties allowed in `power_level_content_override` in the
+# /createRoom API. If any property is missing, its default value will
+# continue to be used. If any property is present, it will overwrite
+# the existing default completely (so if the `events` property exists,
+# the default event power levels will be ignored).
+#
+#default_power_level_content_override:
+#    private_chat:
+#        "events":
+#            "com.example.myeventtype" : 0
+#            "m.room.avatar": 50
+#            "m.room.canonical_alias": 50
+#            "m.room.encryption": 100
+#            "m.room.history_visibility": 100
+#            "m.room.name": 50
+#            "m.room.power_levels": 100
+#            "m.room.server_acl": 100
+#            "m.room.tombstone": 100
+#        "events_default": 1
+
 
 # Uncomment to allow non-server-admin users to create groups on this server
 #

docs/usage/configuration/config_documentation.md

@@ -3315,6 +3315,32 @@ room_list_publication_rules:
     room_id: "*"
     action: allow
 ```
+
+---
+Config option: `default_power_level_content_override`
+
+The `default_power_level_content_override` option controls the default power
+levels for rooms.
+
+Useful if you know that your users need special permissions in rooms
+that they create (e.g. to send particular types of state events without
+needing an elevated power level).  This takes the same shape as the
+`power_level_content_override` parameter in the /createRoom API, but
+is applied before that parameter.
+
+Note that each key provided inside a preset (for example `events` in the example
+below) will overwrite all existing defaults inside that key. So in the example
+below, newly-created private_chat rooms will have no rules for any event types
+except `com.example.foo`.
+
+Example configuration:
+```yaml
+default_power_level_content_override:
+   private_chat: { "events": { "com.example.foo" : 0 } }
+   trusted_private_chat: null
+   public_chat: null
+```
+
 ---
 ## Opentracing ##
 Configuration options related to Opentracing support.