Continue moving content out of docs/model/presence into the main spec; delete model docs that are duplicated

SPEC-25 #comment I've added the details of what the server should do on
receipt of a redaction event. In reality it can do whatever it wants,
and its probably a reasonable implementation to flag it up to a server
admin for verification before actually redacting an event.

Move documented but-unimplemented 'presence idle times' into a new document to contain such features

Nuke the entire 'Typing Notifications' spec section given as they don't exist yet in the implementation

Annotate all the 'TODO' marks as relating to either the specification itself or the documentation thereof

Reörder the specification sections, to move 'Registration and Login' first, where it logically belongs