BK-1296 Implement Editing roles in the Book settings interface

We were needing an interface to manage roles for an specific book. This PR covers that.

Files:
lib/booktype/apps/edit/channel.py
lib/booktype/apps/edit/views.py

don't pass PEP8 test, but it is because a older code. I have fixed that problem on other files but I think we need a new ticket for this two above.

That's it! :)