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

WIP. Please don't hit merge yet.

Following are the use cases

- [ ] 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