Replace room_invite_state_types with room_prejoin_state (#9700)

`room_invite_state_types` was inconvenient as a configuration setting, because
anyone that ever set it would not receive any new types that were added to the
defaults. Here, we deprecate the old setting, and replace it with a couple of
new settings under `room_prejoin_state`.

docs/code_style.md

@@ -128,6 +128,9 @@ Some guidelines follow:
     will be if no sub-options are enabled).
 -   Lines should be wrapped at 80 characters.
 -   Use two-space indents.
+-   `true` and `false` are spelt thus (as opposed to `True`, etc.)
+-   Use single quotes (`'`) rather than double-quotes (`"`) or backticks
+    (`` ` ``) to refer to configuration options.
 
 Example:
 

docs/sample_config.yaml

@@ -1451,14 +1451,31 @@ metrics_flags:
 
 ## API Configuration ##
 
-# A list of event types that will be included in the room_invite_state
+# Controls for the state that is shared with users who receive an invite
+# to a room
 #
-#room_invite_state_types:
-#  - "m.room.join_rules"
-#  - "m.room.canonical_alias"
-#  - "m.room.avatar"
-#  - "m.room.encryption"
-#  - "m.room.name"
+room_prejoin_state:
+   # By default, the following state event types are shared with users who
+   # receive invites to the room:
+   #
+   # - m.room.join_rules
+   # - m.room.canonical_alias
+   # - m.room.avatar
+   # - m.room.encryption
+   # - m.room.name
+   #
+   # Uncomment the following to disable these defaults (so that only the event
+   # types listed in 'additional_event_types' are shared). Defaults to 'false'.
+   #
+   #disable_default_event_types: true
+
+   # Additional state event types to share with users when they are invited
+   # to a room.
+   #
+   # By default, this list is empty (so only the default event types are shared).
+   #
+   #additional_event_types:
+   #  - org.example.custom.event.type
 
 
 # A list of application service config files to use