Improve CoopCycle <-> Weblate integration for translations

[r0xsh]

Having a "link" on untranslated strings that invite users of the software to propose a better/do the translation

[alexsegura]

To stay cheap, not sure we can do much better than a link to Weblate.

Highlighting untranslated message would need a special user role IMHO, to avoid cluttering the interface for normal users.

[alexsegura]

Also, implement a translation provider for Weblate, so that we can maybe use the native Symfony commands to pull translations from Weblate.

Currently, this is done via Git:

git remote add weblate https://translate.coopcycle.org/git/coopcycle/coopcycle-web/
git fetch weblate
git merge weblate/master

[Paul-Eraman-CoopCycle]

I thought to have it only apply to dispatchers, messengers and admin