feat: Implement Post In Group In Webapp

Created by: Tirokk

Pull Request

Implement post in group in webapp.

Issues

fixes #5385 (closed) relates #5059 (closed)

Todo

• post creation with groupId
• post edit with groupId
• post page with groupId
  • side menu is to high
• post listing on group profile
• what is if the user can not post in a group, but puts this group id in the URL?
  • then an error Not Authorized is toasted
• tests: we have to do later
• cleanup: search for Wolle