Configure devise to allow SSO login so we can send accepted users to Canvas

This uses a devise cas gem to use the same sso server as canvas, thus letting both apps use the same database as well as the same actual session. The devise cas gem is incompatible out of the box with the database devise gem we use to track passwords, so I had to patch that above the user class to allow them to live together in that model.

Once that's done, the main site can use the sso path (also in this PR) and redirect to canvas instead of the old /assignments system for the students, giving a better illusion of a single application.
On Thu, Oct 02, 2014 at 06:47:19AM -0700, Brian Sadler wrote:
> What about coaches?  Coaches will have a user_id, no?

Yeah. Actually, now coach vs student is more of a thing inside Canvas than in the platform, but I think the model logic there is still right: a coach has students, a student has a canvas user id...

But all coaches are students of the coach class so I think the distinction here of coach vs student might be an anachronism. The question though is how we'll handle that for things like the resume module - something we'll probably decide later.