Bugfix: Fix duplicated event subscriptions

In some cases it happened that subscriptions to certain topics happened multiple times.
This is problematic in cases where it should happen only once, for example loading a resource which can result in multiple requests and a overlapping state.

This is fixes by introducing the option to unsubscribe a event individually by a given token or for all on a given topic.

https://github.com/owncloud/web/pull/5910
https://github.com/owncloud/web/issues/5875
