Add GitLab Provider from django-allauth

[takotuesday]

Add GitLab as an option to Social Accounts so that users can connect rtd accounts to thier GitLab repositories. There seems to be the demand and use cases, as discussed in #1068.

Once this issue is resolved, integrate the gitlab provider into rtd.

[saily]

Hi, the gitlab provider is implemented, it just needs to be merged, see: pennersr/django-allauth#1231

[ghost]

@saily++

[saily]

merged, let's wait for a new release and then update our version pins in requirements.txt.

[agjohnson]

Commit is pinned with work in #1850, which should also include the commits required here. #1893 addresses bumping to the next release.