Ability to ACL based on event type (SPEC-227)

Submitted by @​matthew:matrix.org
We already have scenarios where we want to give particular users or groups of users permission to send some types of events but not others... or perhaps be allowed to read some types of events but not others. We also need a way to somehow define default permissions for rooms so that users can't create new rooms without these restrictions - the use case for this is that there may be a AS subscribing to said new rooms, which may start picking up events that users inject in virgin rooms

(Imported from https://matrix.org/jira/browse/SPEC-227)

No file, test, or entry point is named. Start by reviewing the Matrix specification sections covering event permissions, room creation, and current access-control rules, then clarify the proposed model for per-event and default room permissions. Done requires an agreed protocol design and corresponding specification changes.

Written by the indexing model from the issue text.