Add the new code approval procedure as a Gitlab merge request template.

We discussed the code approval procedure on the call this morning and referenced the code approval wiki. Gitlab actually supports multiple merge request and issue request templates that are selectable when the MR is created.

This MR adds the code approval checklist (from the wiki page) as a MR template that can be selected when creating or editing the MR on Gitlab. This would allow us to have various "bug-submission" templates once we reach production (in the future).

See here for the relevant Gitlab docs.