PR #6438 does 2 things:
1. Add check-namespace
2. Fix verify script
This PR contains only part 2 (and does some preparations for part 1). The goal of the PR is to make CI succeed on master.
This adds a script `check-strings` that checks whether strings, such as file paths or class names, are valid in files outside of the source code, e.g. in translations or themes. This also adds a verify script to verify `check-strings` on a constant git commit. Both scripts are under CI.
