Merge pull request 'OZG-4948-PostfachOnly' (#432) from OZG-4948-PostfachOnly into master

alfa-client/apps/admin/src/app/app.component.html

alfa-client/apps/admin/src/app/app.component.spec.ts

alfa-client/apps/admin/src/app/app.module.ts

alfa-client/apps/admin/src/app/app.routes.ts

alfa-client/apps/admin/src/assets/mail.svg

alfa-client/apps/admin/src/main.ts

...h-navigation-item/postfach-navigation-item.component.html

...avigation-item/postfach-navigation-item.component.spec.ts

...ach-navigation-item/postfach-navigation-item.component.ts

...pages/postfach/postfach-page/postfach-page.component.html

...es/postfach/postfach-page/postfach-page.component.spec.ts

...c/pages/postfach/postfach-page/postfach-page.component.ts

alfa-client/libs/admin-settings/.eslintrc.json

alfa-client/libs/admin-settings/README.md

alfa-client/libs/admin-settings/jest.config.ts

alfa-client/libs/admin-settings/project.json

alfa-client/libs/admin-settings/src/index.ts

...libs/admin-settings/src/lib/admin-settings.module.spec.ts

...ient/libs/admin-settings/src/lib/admin-settings.module.ts

...fach/postfach-container/postfach-container.component.html