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.

src/domain/IProjectInformation.ts

@@ -4,6 +4,8 @@ import { Version } from '@/domain/Version';
 export interface IProjectInformation {
   readonly name: string;
   readonly version: Version;
+
+  readonly slogan: string;
   readonly repositoryUrl: string;
   readonly homepage: string;
   readonly feedbackUrl: string;