Stop sending push notifications to banned users (#338)

Banning a user deletes their sessions and closes their connections but
keeps their push subscriptions, and Room::MessagePusher chose recipients
by membership alone. A banned user's browser or phone therefore went on
receiving the room name, sender and text of new direct messages,
mentions, and messages in rooms they had set to everything.

Choose subscriptions from active users only. The subscriptions are kept,
so unbanning brings notifications back without the user having to
subscribe again (the client doesn't resubscribe while the browser still
holds a subscription).

Co-authored-by: Marcello Costagliola <176920116+namespaceMarcello@users.noreply.github.com>

app/models/room/message_pusher.rb

@@ -53,9 +53,12 @@ class Room::MessagePusher
       relevant_subscriptions.merge(Membership.involved_in_mentions).where(user_id: mentionees.ids)
     end
 
+    # Banning keeps the user's subscriptions, so that unbanning brings their notifications back,
+    # but nobody who can't sign in should be sent what's said in their rooms meanwhile.
     def relevant_subscriptions
       Push::Subscription
         .joins(user: :memberships)
+        .merge(User.active)
         .merge(Membership.visible.disconnected.where(room: room).where.not(user: message.creator))
     end