GitLab native integration

**Is your feature request related to a problem? If so, please describe.**

I'm using Weblate to access two private Gitlab repositories. Thus I must create a new Gitlab user that has project access to both repositories. Ideally, I could add one `WEBLATE_GITLAB_TOKEN` per project, using the default `oauth2` user. (Similar to #4332, but for repositories within an instance.)

**Describe the solution you'd like**
Allow a `WEBLATE_GITLAB_TOKEN` per repository

**Describe alternatives you've considered**
Creating a new Gitlab user that can access the two repositories. Not ideal because of these reasons:

- I have to ask an admin to create and maintain an extra user.
- If the number of repositories to translate grows, that extra Gitlab user will get access to more and more repositories.