Handle disabled repositories

It is possible for a Git repo to be in a "disabled" state. Being disabled means that the repo cannot be cloned as there are no OAuthCredentials available for them; the frontend displays a red disabled badge on any repo affected in this way.

Currently, retrieving a disabled repository as an internal user causes an AttributeError 'NoneType' object has no attribute 'token', since RepositorySerializer.get_gitlab_token does not take disabled repos into account: https://sentry.io/organizations/teklia/issues/1747325948/

The API should instead just return None and the Git import task should fail gracefully. It is not impossible that this issue has been overlooked in different places.