Add webhook for "/ci-test" tasks

For compatibility with the old frontend and the way /ci-test jobs are triggered, I added a the route /jenkins/ that accepts POST requests authenticated with the right X-Gitlab-Token header (the value of the server environment variable WEBHOOK_KEY).

When the JSON data passed looks like a MR message with the string /ci-test in it, the document is added to a queue in CouchDB, where a matching Jenkins job (new-ci-test-poll) will pull it and trigger the normal chain (gitlab-mr job).

It's a very poor messaging system (the same used for the new release system and !92 (merged)), but it's a practical while the new nightly builds system is developed.

Closes #42 (closed)