Change GithubIdentity into RepositoryIdentity that works for GitLab and Bitbucket

[andrew]

Follow on from #1169 and #1344

We have removed the GitHub specific bits for repository owners but still have a number of functions for logged in users that only work for GitHub, most of which are tucked away in https://github.com/librariesio/libraries.io/blob/master/app/models/concerns/github_identity.rb

This needs to be moved onto their own classes which should be available via the identity model, which a user can have multiple of, this should enable GitLab and Bitbucket users to:

• Sync their org/group/team memberships and repositories they have access to
• Monitor repository dependencies

n.b. private repo functionality may need new oauth apps with extra permissions for GL/BB

[andrew]

Moving this to the Backlog as we'd still like to implement it but can't see that happening in the near future.