[WIP] GitLab integration

[raethlein]

What kind of change does this PR introduce?

• [ ] Bugfix
• [X] New Feature
• [ ] Feature Improvment
• [ ] Refactoring
• [ ] Documentation
• [ ] Other, please describe:

Description:

This PR adds integration for GitLab and other repositories that are based on GitLab (e.g. heptapod). It adds similar information as the GitHub integrator, but the the GitLab GraphQL seem to not provide all the information such as the project's license. Hence, this information is missing.

Adding libio or GitLab's Rest API to fetch some more information should be evaluated and probably added.

In general, it might make sense to refactor the code base from the perspective that it does not rely on GitHub - there are some checks in the code that require GitHub - but rather a code repository id.

An example project entry for the integration to test with is:

  - name: openpyxl
    gitlab_id: openpyxl/openpyxl
    api_url: https://foss.heptapod.net/api/graphql
    pypi_id: openpyxl
    conda_id: openpyxl
    dockerhub_id: openpyxl/openpyxl-ci
    docs_url: https://openpyxl.readthedocs.io/en/stable/
    category: data-loading

Checklist:

• [X] I have read the CONTRIBUTING document.
• [X] My changes don't require a change to the documentation, or if they do, I've added all required information.

[lukasmasuch]

Thanks! Looks good for me, I added a few comments/questions above.

[lukasmasuch]

I added the gitlab_id to the documentation.