Assign SSH keys per project (currently: per user account) (BB-3019)

Problem:

I enable access to a project on a server, where other people also have access to. The assigned SSH key leaves open access to my other repositories. Current solution is to register another account just to handle this scenario, but IMO it's nonsense to have a separate project owner for every production server. Not only because you have to register, confirm email, assign admin, etc. every time, but because these project don't appear in 'My repositories' (btw, it'd be nice if projects I'm admin of appeared in a more accessible section)

Example solutions:

• upload SSH keys in project admin panel, not user admin panel
• leave as it is and add checkboxes with a 'restrict access' option where you select which project would this key give access to