Add git refs

What does this MR do?

Makes the information added by nitrile!71 (merged) available through the API. On the frontend, a hidden link is added to version numbers on /pkg/PACKAGE which brings the user to GITLAB_URL/tree/REF.

Changes to public APIs

• Adds an optional git_ref to package version targets on /api/packages and /api/packages/PACKAGE
• To update, the new column must be added to the database manually.

Author's checklist (required)

• Intermediate commits compile (use git rebase -i main if not)
• Newly added code has a style consistent with the rest of the repository
• Newly added code is documented (guidelines)

Related issues