OZG-6988 refactor postfach setting handling (as an example)

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

alfa-client/libs/admin/postfach-shared/src/index.ts

...dmin/postfach-shared/src/lib/postfach-resource.service.ts

...ient/libs/admin/postfach-shared/src/lib/postfach.model.ts

.../libs/admin/postfach-shared/src/lib/postfach.providers.ts

...nt/libs/admin/postfach-shared/src/lib/postfach.service.ts

.../lib/postfach-container/postfach-container.component.html

...rc/lib/postfach-container/postfach-container.component.ts

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

...stfach-container/postfach-form/postfach-form.component.ts

.../postfach-container/postfach-form/postfach.formservice.ts

...ibs/admin/settings-shared/src/lib/settings-fom.service.ts

...nt/libs/admin/settings-shared/src/lib/settings.service.ts

...lient/libs/admin/settings-shared/src/lib/settings.util.ts