WIP: Matrix chat integration

Planned features:

• rooms (public + private) are created for each group (via grouprisectl matrix_chat_manage create-rooms)
• configured room details:
  • title
  • description (topic)
  • public / private (via the Matrix preset: private_chat and public_chat)
  • alias address (#group:example.org and #group-private:example.org)
  • group logo (synchronization is too hard to implement due to required mxc:// URLs)
• configured user details:
  • avatars (not easily doable - see group logo above)
• all members are invited to the rooms (via grouprisectl matrix_chat_manage invite-room-members)
• signals / hooks for various events:
  • create room after a group is created
  • invite/join after a user subscribes/joins a grouprise group
  • leave after a user unsubscribes/leaves a grouprise group
  • send grouprise contributions (i.e. their URLs) to the relevant rooms
• create a "listen to everything" room
  • similar to the grouprise frontpage:
    • no human user may speak
    • everyone is subscribed by default (but may leave)
    • all public contributions are broadcast
• configurable details:
  • custom (external) matrix ID configurable for each user account
  • optional link to matrix room displayed the group page (can be hidden via group settings)
• element web interface:
  • configure single-sign-on login
  • configure grouprise/site background in element's login form
  • configure grouprise wording
• allow login to chat via the grouprise user:
  • single-sign-on via CAS-Server
    • Since the active grouprise session and the single-sign-on authority are not connected (thus element web is the only SSO client), this requires a manual login with the grouprise credentials.
  • authenticate to matrix server with grouprise credentials
    • this requires a manual login (despite the ongoing grouprise user session)
• create deb packages