Feature/edit metadata authorization

I assume that pds_handle contain user group name(s) and thus we don't need to add roles. To reflect this I added customized method "groups" to user model. If the assumption is incorrect I will fix it and edit roles file.

@ekate Please describe what's going on here and why it's necessary as comments in the code. Also, make sure that sending a method from the `controller_path` doesn't open us up to any security risks.