Provide facility to manage multiple forks of the same upstream repository

[ddimtirov]

It is expected that in a single organization, we would have more than one individual contributors to the same upstream repository.

Example: Linda in Lisbon, Dave in Dublin, and Taro in Tokyo have all forked Node.JS and independently have worked on patches to contribute. GitProxy should recognize that these 3 upstreams have a common ancestor and allow to manage the approvals and audit of these together.

The most straight-forward implementation approach is to be able to manually link multiple proxy-repos as a group.

As a separate service, we can have repo-similarity detection service, looking for common branches, containing identical commit chains greater than a size-threshold. It is an open problem how to distinguish legit forks (i.e. OpenSSL vs. BoringSSL)

[kriswest]

I believe this issue and #511 are connected. The repository name is being used as a unique key in various parts of the codebase which causes errors when working with multiple forks of the same project - the wrong project will often be retrieved and changes applied to it. By switching to the repository URL for uniquely identifying and retrieving projects (as @msagi's PR #545 attempts to do) you can support both multiple forks of the same repo and non-github repos (by also cleaning up hardcoded github.com urls).

I'll open a separate issue to try and tie all these things up - and then hopefully a PR to resolve