Add GitLab integration for scaffolder

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

plugins/techdocs-backend/package.json

@@ -30,7 +30,7 @@
     "express": "^4.17.1",
     "express-promise-router": "^3.0.3",
     "fs-extra": "^9.0.1",
-    "git-url-parse": "^11.1.3",
+    "git-url-parse": "^11.2.0",
     "knex": "^0.21.1",
     "node-fetch": "^2.6.0",
     "nodegit": "^0.27.0",