Collaborators management

I'm making a pull request, so I can comment on this more easily. You can still push further fixes to this branch.
`users` is going to be an array of strings, just take a look at sample params:

```
{"utf8"=>"✓",
 "authenticity_token"=>"t34seE07epKGSpcXVf2K6phAaXmPuffqmkRqf2HKEyY=",
 "user_id"=>["2", "3"],
 "commit"=>"Share"}
```

So those are not users, but user_ids.