Allow per-room profile to be used for server notice user

This applies even if the feature is disabled at the server level.
The server notice not being a real user it doesn't have an user profile.

Signed-off-by: Mathieu Velten <matmaul@gmail.com>

changelog.d/8799.bugfix

@@ -0,0 +1 @@
+Allow per-room profile to be used for server notice user.

synapse/handlers/room_member.py

@@ -347,7 +347,17 @@ async def update_membership_locked(
             # later on.
             content = dict(content)
 
-        if not self.allow_per_room_profiles or requester.shadow_banned:
+        is_requester_server_notice_user = False
+        if (
+            self._server_notices_mxid is not None
+            and requester.user.to_string() == self._server_notices_mxid
+        ):
+            # allow the server notices mxid to set room-level profile
+            is_requester_server_notice_user = True
+
+        if (
+            not self.allow_per_room_profiles and not is_requester_server_notice_user
+        ) or requester.shadow_banned:
             # Strip profile data, knowing that new profile data will be added to the
             # event's content in event_creation_handler.create_event() using the target's
             # global profile.