Automatically & continuously import updated translations from Hosted Weblate

Goal: spot and report PO files problems early, instead of in the critical path of the release process (when it's too late to fix them).

As a temporary workaround, we have !241 (merged). But now that we have trusted GitLab CI runners, instead we should have a job that imports updated PO files from Hosted Weblate, commits, and creates a merge request. Then, developers could merge that merge request as-is if, and only if, it passes the lint-po CI job.