Reduce response size 

This was discussed in #1419 

It is only for batchUpdate, other routes will still send down the whole user object (like update user)

- Set wasModified = true always when ?v is missing
- Only send the _v when user is not modified, it works also without a response at all but I tought it could be needed by the local object to know the user version. @lefnire let me if it's not needed and i'll make it send a 204 http code with no response (but then I believe we'll need to update the User Service in the mobile app or it'll break with an empty response)

Tested and working.

Maybe when we'll revisit the api (the v2 @lefnire talked about in #1419) we can also optimize the other routes. As for the website now it sends down only ~15bytes of data.