Add transitional GitLab domain source (no cache)

Description

This merge request adds a transitional GitLab domains source.

• It does not have caching implemented yet, what we plan to do in a separate merge request.
• The purpose of this code is to make a new GitLab client testable for some subset of domains that we configure via ENV.
• It also implement a new case in which the domain source can not retrieve a domain information, in that case we need to present a new error page, similarly to what we show in Workhorse when it is not able to reach GitLab.

Closes #254 (closed)

/cc @ayufan @nicholasklick