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.
You can already see in your collaborations controller that you need a wiki object. So, since you need a wiki, the URL needs to contain an ID of the wiki. Otherwise, how are you going to look up the wiki in the DB?

Moreover, '/wikis/edit' is a bad name for this action. Why not just go the obvious route and use '/collaborations/add_collaborators'? Since you will need to put the wiki ID there somewhere, then maybe '/wikis/:id/collaborations/add_collaborators' is a sensible route?