Add timeout to user and group deletion notification

Added timeout in DeleteHandler to auto-delete after a delay.

Fixed issue where OC.Notification.hide() was called twice in a row when
deleting multiple entries, causing the second notification to disappear.
Fixed issue where "undo" click event handler was registered multiple
times when calling setNotifications() twice.
Added JS unit tests for the DeleteHandler class.

Refix undo users, groups feature

Timeout is now cleared in cancel().

Fixed click handler name for "undo" to be able to work with multiple
DeleteHandler instances (in our case one for users and one for groups)
so that there is no conflict.

Backport of 0d9f24a0efef20b9041e40817b10822a4700532d from master