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.
I recommend using `for (var i = 0; i < result.data.groups.length; i++)` instead. Some libraries might break for loops with `in`. [optional change]

Another alternative is to use underscore JS:
```
_.each(result.data.groups, function(gid, i) {
   // ...
});
```

Just saying, no need to change now :smile: