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.

tests/unit/shared/Stubs/ProcessEnvironmentStub.ts

@@ -1,7 +1,10 @@
-export function getProcessEnvironmentStub(): NodeJS.ProcessEnv {
+import { VueAppEnvironment } from '@/application/Parser/ProjectInformationParser';
+
+export function getProcessEnvironmentStub(): VueAppEnvironment {
   return {
     VUE_APP_VERSION: '0.11.3',
     VUE_APP_NAME: 'stub-name',
+    VUE_APP_SLOGAN: 'stub-slogan',
     VUE_APP_REPOSITORY_URL: 'stub-repository-url',
     VUE_APP_HOMEPAGE_URL: 'stub-homepage-url',
   };