WIP : Increment / Decrement User counts for several use-cases.

WIP. Please don't hit merge yet.

Following are the use cases

- [x] Increment Everyone count by 1 when a user is created
- [ ] Decrement, if user is deleted
- [ ] Increment / Decrement count(s) of groups under him he is created / deleted.

cc @blizzz @jancborchardt 
The data attribute is actually set for this https://github.com/owncloud/core/blob/master/settings/js/users/groups.js#L33