Support other VCS providers than GitHub for connecting projects to repositories

[fxjordan]

A Requirements Bazaar project can be linked to a GitHub repository, so users can easily create new issues from requirements and the backend service receives GitHub updates (e.g., issue closed, release published, pull request merged) which can be used to update a requirements state.

Of course, not all code is hosted on GitHub, so we should also allow for additional providers like GitLab.