Implement rejected events (#1426)

* WIP Event rejection

* Still send back errors for rejected events

Instead, discard them at the federationapi /send layer rather than
re-implementing checks at the clientapi/PerformJoin layer.

* Implement rejected events

Critically, rejected events CAN cause state resolution to happen
as it can merge forks in the DAG. This is fine, _provided_ we
do not add the rejected event when performing state resolution,
which is what this PR does. It also fixes the error handling
when NotAllowed happens, as we were checking too early and needlessly
handling NotAllowed in more than one place.

* Update test to match reality

* Modify InputRoomEvents to no longer return an error

Errors do not serialise across HTTP boundaries in polylith mode,
so instead set fields on the InputRoomEventsResponse. Add `Err()`
function to make the API shape basically the same.

* Remove redundant returns; linting

* Update blacklist

sytest-blacklist

@@ -40,11 +40,6 @@ Ignore invite in incremental sync
 New room members see their own join event
 Existing members see new members' join events
 
-# Blacklisted because the federation work for these hasn't been finished yet. 
-Can recv device messages over federation
-Device messages over federation wake up /sync
-Wildcard device messages over federation wake up /sync
-
 # See https://github.com/matrix-org/sytest/pull/901
 Remote invited user can see room metadata
 
@@ -56,8 +51,8 @@ Inbound federation accepts a second soft-failed event
 # Caused by https://github.com/matrix-org/sytest/pull/911
 Outbound federation requests missing prev_events and then asks for /state_ids and resolves the state
 
-# We don't implement device lists yet
-Device list doesn't change if remote server is down
-
 # We don't implement lazy membership loading yet.
 The only membership state included in a gapped incremental sync is for senders in the timeline
+
+# flakey since implementing rejected events
+Inbound federation correctly soft fails events