This adds a GitLab integration for the scaffolder backend.
We're introduceing a preparer and a publisher for GitLab so that we can
read templates from GitLab and publish them to a configured GitLab
instance. The two instances don't need to be the same. For instance,
templates could be public on gitlab.com, but the created repos will live
in a hosted GitLab somewhere else.
The publisher gets its own config object in `app-config.yaml` where the
target instance and token can be specified.
The service catalogue defines both `gitlab` and `gitlab/api` as
processors. They are both handled by the same preparer.
Closes#2372
Björn Marschollek
55542797a9