Enhancement: Add SSE to get notifications instantly

We've added SSE to the notifications which allows us to be notified
about new notifications instantly and from the server without polling
every few seconds.

https://github.com/owncloud/web/pull/9451
https://github.com/owncloud/web/pull/9654
https://github.com/owncloud/web/pull/9753
https://github.com/owncloud/web/issues/9434
https://github.com/owncloud/web/issues/9635
