Followups: Don't send all room userids in signaling server requests

[nickvergessen]

• Handle updates of many participants that can still cause an error
• Include inactive participants in the list of changed participants in BackendNotifier::participantsModified()
• Notify missing room changes, like avatars or in call status
• Notify inactive participants when a room is modified

Originally posted by @danxuliu in #16759 (review)

[Antreesy]

Changes to be considered on client side:

Signaling.Standalone.prototype.processRoomParticipantsEvent -> participants -> update:

• event now contains only the changed participants instead of the full list of participants in the room;
• participants leaving a call are still signaled by their own message with inCall: 0;
• participants joining a call should be as before, (but source of truth is now HPB, not PHP).
• participants that are offline but changed (e.g. promoted to moderator, permissions change) are not signaled anymore (rely on PHP request or chat sytem message)
• full list after guest cleanup is no longer emitted.
• all events only sent to participants currently in the room (as before)

Signaling.Standalone.prototype.processRoomListEvent -> roomlist -> ...:

• update with room properties (e.g rename, description, lobby) only sent to participants currently in the room (rely on PHP request with modifiedSince)
• update with invite/disinvite (user added/removed, guest joined/left, self-leave) only sent to participants currently in the room (rely on PHP request with modifiedSince)
• disinvite with reason: "deleted" only sent to participants currently in the room (rely on PHP request with modifiedSince=0 - full fetch)

(Analysis is AI-assisted)