Use CI to create language files

[selmf]

We should use our new CI system to run lupdate on our source code whenever we do a (pre)release. We should publish the generated translation source files as a separate download alongside the release.

This has several advantages:

• we don't have to worry about our translation files being up to date
• translators can just grab the new sources and start working once we entered the release process
• the entry threshold for creating or improving a translation is considerably lower

We'll need to think about how we'll be re-integrating the generated files into git though.