Merge '1-upgrade-to-angular-15' into 'main'

This merge request upgrades the application to Angular 15, including upgrades for the Angular core dependencies, Angular Material and Apollo Angular.

Additionally, the application was migrated to the new MDC-based Angular Material v15 components. This changes the general visuals of the application slightly, but was aligned with the old styles. Some other visual issues were fixed in the process, too, e.g. making the navigation bar responsive.

Closes #1 (closed)