increment/decrement group and user counts when necessary

supersedes https://github.com/owncloud/core/pull/9272, Increment / Decrement User counts for several use-cases.

- [x] Increment Everyone count by 1 when a user is created
- [X] Decrement, if user is deleted
- [X] Increment / Decrement count(s) of groups under him he is created / deleted.
Looks like this is doing the same as `modEveryoneCount` but with a specific group.
You could have created a function that does it for "any" group, and then call that function with the $li from the "everyone" group when needed.