.gitlab-ci.yml: start rails spec and cleanup automatically

Previously when the rails pipeline was introduced, there was one manual job that kicked off a series of jobs to run the rails specs on a branch, and then clean up that branch. However, we ran into issues because those jobs would run during merge trains and scheduled pipelines, which would lead to failures.

Add if conditions so that these jobs are only run for merge requests. This way, scheduled pipelines and merge trains shouldn't be affected. In the future we probably will want to find a way to enable it for merge trains.