Trigger auth flow through GitHub App
Created by: pjlast
This PR implements CLOUD-205.
If a user belongs to an organization that has the GitHub App feature flag enabled, the GitHub App authorization flow is used to create the user's connection to GitHub.
As stated in CLOUD-205, what isn't handled is the functioning of the callback URL (this is future work). This is why there is an Auth error once the app navigates back to Sourcegraph.
https://user-images.githubusercontent.com/6427795/152921270-917f251a-e7e3-4691-944a-8a56504bf15b.mp4