build: create a script to detect and fix symlinks based on po files

[alexgarel]

We have links that are translated, like "footer_who_we_are_link".

When they are translated, we should check that we have a corresponding symlink <msgstr>.html --> <msgid>.html (eg wer-wir-sind.html --> who-we-are.html) in the corresponding language folder.

The script may live in openfoodfacts-server repository, using the openfoodfacts-web link in the docker container.

It may log bad links and create missing one.
It may also warn if they are concurrent, old links pointing to same file.