CI: Add checks for `TODO` in MRs
See !357 (comment 335165) and the following comment. It would be good to automatically fail MRs which contain
TODO anywhere in them.
FIXME should be allowed, but only if there’s a URI on the same line, or something like that. Probably don’t want to restrict it to a GitLab issue URI, since people might want to make a FIXME conditional on a bug in a different bug tracker, or something like that.