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.
This file should be in the subscription branch. To fix that:

1. checkout collab branch
2. copy app/views/subscriptions/_cancel.html.erb file somwhere safe
3. git rm app/views/subscriptions/_cancel.html.erb
4. commit
5. checkout subscriptions branch
6. create app/views/subscriptions/_cancel.html.erb file and put the saved contents into it
7. git add app/views/subscriptions/_cancel.html.erb
8. commit