Don't spam pending invites every time NeoChat is started

Currently the way we show invite notifications is sub-optimal. We did it
during the initial room state load, which meant it shows an invite
notification *every time* you opened NeoChat. This gets annoying very
quickly if you have any pending invitations you don't want to take
action on just yet.

Instead, let's handle this in NotificationsManager directly, and also
remove some scaffolding now that it isn't plumbed through
NeoChatRoom/NeoChatConnection.

src/notificationsmanager.cpp

@@ -127,9 +127,8 @@ void NotificationsManager::processNotificationJob(QPointer<NeoChatConnection> co
         }
         auto sender = room->member(notification["event"_L1]["sender"_L1].toString());
 
-        // Don't display notifications for events in invited rooms
-        // This should prevent empty notifications from appearing when they shouldn't
         if (room->joinState() == JoinState::Invite) {
+            postInviteNotification(qobject_cast<NeoChatRoom *>(room));
             continue;
         }