Remove gitlab integration

Refs https://redmine.teklia.com/issues/5720

We do not need to integrate with gitlab (or any other git provider), as we plan on not building the worker in arkindex at all.

This issue covers the removal of :

• python-gitlab dependency
• gitlab webhook
• Repository.hook_token field
• CreateExternalRepository & ListExternalRepositories endpoints
• gitlab provider
• update_repositories_hooks & update_repositories_refs django command