Support multiple code repositories per project

[fxjordan]

Currently, only one code repository can be linked to a project. However, many projects (e.g., Requirements bazaar itself https://requirements-bazaar.org/projects/2) consist of multiple components which may be managed in different git repositories.

Proposal:

• Support linking multiple repositories to the same project
• Allow for configuring a default repository for each category, which is used to create new issues from requirements