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 
I'd prefer if we could store the number in a data attribute, maybe in the `ul` header as `data-usercount`.
This is because parsing text values with `.text()` can be unreliable and likely to break.