Implement MSC3952: intentional mentions (#9983)

Implements the intentional mentions feature of MSC3952 (behind
a labs flag).

If enabled, this will send an org.matrix.msc3952.mentions property
on events that will contain the user IDs and/or whether the room is
being mentioned. These mentions also gets propagated via some
custom behaviour for replies and edits.

src/components/views/rooms/wysiwyg_composer/utils/createMessageContent.ts

@@ -125,6 +125,8 @@ export async function createMessageContent(
 
     const newRelation = isEditing ? { ...relation, rel_type: "m.replace", event_id: editedEvent.getId() } : relation;
 
+    // TODO Do we need to attach mentions here?
+    // TODO Handle editing?
     attachRelation(content, newRelation);
 
     if (!isEditing && replyToEvent && permalinkCreator) {