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 
Even better than a data attribute would simply to store the value in a private attribute like `UserList._usersCount`. Then it's very easy to increment it.

Then add a method `UserList.setUsersCount(count)` that will update that value in `_usersCount` and also update the DOM.