Show an error screen when the homeserver is missing MSC features

[robintown]

We could do this to (rather forcefully) nudge homeserver admins to turn on support for delayed events.

MSCs we do care about are:

experimental_features:
    # MSC3266: Room summary API. Used for knocking over federation
    msc3266_enabled: true
    # MSC4222 needed for syncv2 state_after. This allow clients to
    # correctly track the state of the room.
    msc4222_enabled: true

# The maximum allowed duration by which sent events can be delayed, as
# per MSC4140.
max_event_delay_duration: 24h

Questions to answer:

• Given a mismatch in required enabled MSCs do we soft-fail or hard-fail? (thinking of delayed events)
• soft-fail candidates
  • For msc3266 this would only affect SPA guest user access.
• hard-fail candidates
  • msc4222: hard fail since we can't guarantee proper operation
  • btw. can we also check rate limiting client side? or only in case we are running into it?