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 As far as I can tell this shouldn't work. You want to authorize on the resource not the id,

So `authorize! :update, @item` would be the statement. However, you can add a `load_and_authorize_resources`, which will do the loading (i.e. `@item = Item.find(params[:id])`) and the authorizing for you. See here for more: https://github.com/ryanb/cancan/wiki/authorizing-controller-actions