check missing translations in CI

[deltragon]

Currently, it is easily possible to add a new translatable string in Python, without also adding it to the .pot file or without adding it to the individual languages' .po files by running update-po.sh. It should be possible to write a github action/extend the existing action to ensure that running xgettext/pygettext.py would not add any new strings to the .pot file, and that update-po.sh would not add any new strings to the .po files. (The existing validate_po.py already uses polib, we could reuse that for this check too.)

This would avoid issues like https://github.com/slgobinath/SafeEyes/issues/569 (fixed by https://github.com/slgobinath/SafeEyes/commit/603e1d744510862de2e981ffa5393d6bea2fae7e) in the future.