Initial attempt at GitLab CI

Initial attempt at GitLab CI

CI currently runs on merge requests *only*.

- Use standard[rb] for linting. Standard differs pretty significantly
  from the upstream rubocop configuration, most notably by using
  double‐quotes for all strings instead of only for strings with
  interpolation. To minimize the amount of change required, linting is
  only run on changed files, thanks to an inlined deno script which
  imports JSON from the GitLab API. Linting is also allowed to fail
  without impacting an MR’s mergability.

- Run rspec tests.

- Do not do any CI on JavaScript or (S)CSS. Tests currently do not pass
  for the GlitchSoc frontend and we aren’t planning on spending
  extensive development energy here.