Add check-strings (#6379)

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.

.github/workflows/check-strings.yml

@@ -0,0 +1,19 @@
+name: check-strings
+
+on: [push]
+
+jobs:
+  build:
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: Install python-tinycss2
+      run: sudo apt-get install -y python3-tinycss2
+    - name: Update submodules
+      run: git submodule update --init --recursive
+    - name: Verify check-strings script
+      run: tests/check-strings/verify
+    - name: Run check-strings
+      run: tests/check-strings/check-strings