GitLab Integration

[asaphko]

We currently have a stable and highly used GitHub integration. We're hearing from existing and potential customers that they would also like to connect their GitLab repositories with Flagsmith.

This issue is a Spike, with the following expected outcomes:

1. Review the existing GitHub integration code - I've been told it's old and messy - and asses if and which refactoring is needed to bring it up-to-date.
2. Consider a modular approach to the above - we now want to add support for GitLab, but will probably want to add support for additional services in the future, such as BitBucket.
3. Plan and estimate the required work for the above + adding support for GitLab to match our existing GitHub integration functionality and capabilities.
4. Plan and estimate the required work for adding GitLab support for our newly released GitHub Code References feature.

The goal is to provide a same level of service, capabilities, and features for users who use GitLab instead of GitHub.