Store optional git branch for worker versions

Refs https://redmine.teklia.com/issues/8581

We removed git references earlier, but we still need to track git branch along git tags (already supported) for worker versions.

Actions:

• add a nullable charfield WorkerVersion.branch (size 512)
• extend WorkerVersion.tag size to 512
• make sure both fields are editable through admin
• branch is unique when not null by worker (same as tag)
• worker version endpoints (create/update/partial update) must support a new optional branch field
  • when the field is specified, all mentions of that branch on a previous version of that worker must be reset to null (no conflict ever)
• ListWorkerVersions must expose the branch