Add ability to sign up with Github

[joshsmith]

Reference notes from Rails repo

[begedin]

Using the facilities we have, here's what we should be able to do

• Implement ember side - Github SignUp/SignIn code-corps-ember#1373

• On the API side

  • the connect process should create a user if it's missing
  • the connect route should sign the user into the con
  • the connect route should not require authentication

Alternatively, If it turns out the sign-up/-in process is too different from the plain connect process, we could offer alternative routes and alternative redirect_to urls on the ember buttons.

[joshsmith]

We would also want to connect any existing tasks/comments that came from GitHub to our now GitHub-connected Code Corps user.

[begedin]

The connect process already does this with installations, so we ought to probably do the same with the other things. However, that is not strictly a part of GitHub sign in/sign up, but a part of GitHub connect generally, so I think it's due for a separate issue (or a couple).

[joshsmith]

Yeah, mind opening some relevant ones?

[begedin]

• Handling bot user when handling github webhook #939 is relevant to this, since the current sync process seems to be buggy.
• When connecting user to github, associate tasks #941 and When connecting user to github, associate comments #942 were created for connecting tasks and comments, respectively