OZG-6620 place signatur in postfach mails

alfa-client/libs/postfach-shared/src/lib/postfach.model.ts

...ent/libs/postfach-shared/src/lib/postfach.service.spec.ts

alfa-client/libs/postfach-shared/src/lib/postfach.service.ts

alfa-client/libs/postfach-shared/test/postfach.ts

.../lib/postfach-mail-form/postfach-mail-form.component.html

...b/postfach-mail-form/postfach-mail-form.component.spec.ts

...rc/lib/postfach-mail-form/postfach-mail-form.component.ts

.../lib/postfach-mail-form/postfach-mail.formservice.spec.ts

...h/src/lib/postfach-mail-form/postfach-mail.formservice.ts

...er/postfach-nachricht-reply-editor-container.component.ts

.../ui/editor/textarea-editor/textarea-editor.component.html

...ib/ui/editor/textarea-editor/textarea-editor.component.ts