.gitlab-ci.yml: Add rails job

We often make changes to the Gitaly API that affects downstream Rails pipelines, but we don't yet have the ability to easily trigger a job to run the full rails suite with the current Gitaly commit sha as GITALY_SERVER_VERSION. Add a series of jobs that:

1. creates a new branch off of gitlab-org/gitlab's master branch (since we don't have permission to start jobs off of the master branch), and modify the GITALY_SERVER_VERSION file on it.
2. run the rails suite
3. delete this ephemeral branch

blocked by: gitlab!124337 (merged)

note: gitlab!124782 (merged) will need to be merged as currently setting GITALY_SERVER_VERSION for the rails tests breaks the tests. But, we can still merge this one.