M11.3 Switch to Spaces by default

[t3chguy]

• Load Spaces by default instead of Communities
• Temporarily support Communities in Settings
  • Add a toggle to Preferences
  • If linked to a community (prefixed with +) show a relevant tooltip ‘Enable Communities in Preferences’
  • If trying to load a community (e.g. by slash command) show a new full screen view (leveraging the space preview?) with
    • Preview metadata on the community
    • Micro copy on how to enable Communities in Settings
• In future, sunset support for Communities

[nadonomy]

@novocaine this is in the Design Review column but has been approved (matrix-org/matrix-react-sdk#6594).

I assume we're waiting until we're ready to exit beta to merge this? Pls can you move the card to wherever you think is best in the board?