add-forge-now: Trigger gitlab pipeline when a request is accepted

Related to #4760 (closed)

The purpose of this modification is to trigger a Gitlab pipeline to process add-forge-now request when a request status is changed to accepted.
There are several points I have trouble with:

1. I don't know where to store the Gitlab pipeline token.
   During my tests in swh-environment I stored it in an environment variable declared in the docker-compose.override.yml.
   The Gitlab URL should probably be stored in a variable too.
2. For now these modifications should just be available and active only in the staging environment while the whole process is tested.
3. When I realized tests in swh-environment the webapp window often froze (I don't know if it's my POC configuration or the code modifications).