Add Invitation model and Habitat/Assembly Invites

Features:

• Invitation Model
  • Add Forms for Habitat Invitations
    • Add Invite for Assemblies to Habitats
  • Update Habitat children display
  • Add Invitation displays to Assembly
    • Styling verbessern

• Add rules package for advanced permissions
  • Add rules for Assemblies
• Translations
• Checks for allowed states (Already exists, recently rejected)
• Tests

Replaces !994 (closed)

Fixes #568 (closed)