feat(workflow): add string filter for languages

[michael-smt]

Proposed changes

This adds a new setting to the language workflow of a project to exclude units from translation.

With regional languages it can be useful to only translate a subset of units. This avoid duplicating translations which have to be kept up to date, instead relying on a fallback in the localization framework (e.g. #2320 (comment)). By limiting translation to only the relevant units it is clear what is region specific and the progress can be properly reflected. See also #3412.

For example first you label all units that should be regionalized for British English with a label like uk-regional. Then configure the project's en_GB language workflow with the filter label:uk-regional. Only these units are available for translation to en_GB.

Checklist

• Lint and unit tests pass locally with my changes.
• I have added tests that prove my fix is effective or that my feature works.
• I have added documentation to describe my feature.
• I have squashed my commits into logic units.
• I have described the changes in the commit messages.

Other information

This PR is far from complete and the filter will need to be added in a lot of places. I first wanted to get a feeling if this feature makes sense.

Also the naming might not be ideal...

[nijel]

I'm not really convinced this is a practical approach, as this opens up several consistency issues. Two ones I was able to figure out quickly:

• This will hide the strings from the translating UI while they still being counted for in the statistics, making it impossible to complete the translation.
• The filtering will apply only at certain levels (when language settings apply) and not other (project- or site-wide search). So you will find the string in the site-wide search, and clicking on it will tell you that the string does not exist.