Automatically check PO files in all our repositories

This could be done in a number of ways:

on our Jenkins, which was the initial plan, that’s still expressed in the ticket relationships; pros: we already have most of the building blocks in place; cons: only very few people can do this work and the way test results are published is not user friendly at all for people who don’t have access to the Jenkins interface and need to navigate https://nightly.tails.boum.org/check_PO_master/
via #15362 (closed); pros: some people are actively working in this area; cons: AFAIK that’s not a complete CI system that already knows how to send notifications, publish test results and their history on the web, this sort of things, and I’d rather not see us reinvent that wheel
with GitLab CI, e.g. on Salsa or 0xacab; pros: super simple to set up, test results & history are published nicely for everyone, more of us know how to manage that compared to Jenkins; cons: I’m slightly concerned about the confusion that can be caused by adding another CI platform to our toolboxJenkins does

Original created by @intrigeri on 8358 (Redmine)

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