Change subtitle heading to new slogan

- Unify reading subtitle/slogan throughout the application.
- Refactor related unit tests for easier future changes.
- Add typed constants for Vue app environment variables.

package.json

@@ -2,6 +2,7 @@
   "name": "privacy.sexy",
   "version": "0.11.4",
   "private": true,
+  "slogan": "Now you have the choice",
   "description": "Enforce privacy & security best-practices on Windows, macOS and Linux, because privacy is sexy 🍑🍆",
   "author": "undergroundwires",
   "scripts": {