Add connect to Github in the app UI

[apburnes]

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To allow users to see their sites or add new sites, they will need to get a sign in and get Github token to associate to their Pages user.

Before moving to UAA authentication, the app was getting a Github token whenever the user logged into the platform. Now that we are moving to UAA auth, users are no longer getting the token.

To Do:

• [x] Create an interface to connect your pages account to Github
• [x] Modify or adapt the existing Github authentication endpoints to get a token for a user.