Remove variable event types; clarifications
* Clarify that this proposal includes history - see MSC3672 for EDUs
* Note dependency on MSC3267 for reference relationships
* Remove variable event_type - instead use state_key
* Propose state_key prefixes inspired by MSC3757 and MSC3779
* Update links to matrix-spec-proposals
* Clarify how to stop sharing, and when a share is stopped
* Mention the need for redactions to work
* Clarify the use of milliseconds for all timing
* Mention implementations
