feat(other): `@all` mention for network admins

Created by: Tirokk

🍰 Pullrequest

@all mention for network admins

Issues

• fixes #6593

Todo

Only for network admins:

• ?have @all mention in comments as well?
  • at the moment it is this way in the
    • webapp
    • backend
• webapp – editor:
  • add the @all mention to the mention menu:
    • add it at the beginning of the menu and seperate it by a seperation line
  • if the user choses @all it is placed as a link in the editor:
    • ?what the link and id(?) shall be?
• backend:
  • blacklist all from the slugs?
  • check that the sender is admin
  • if the @all mention is in the post content:
    • send all users a notification:
      • ?sender is the admin?
• cleanup: search Wolle