fixes #541

fixes #634

The actual reuseItems is still blocked on a few upstream bugs.

Signed-off-by: Stuart Mumford <stuart@cadair.com>

As of 0db4d71e (Prevent edits of
unsent messages), messages that are edits of (or replies to) unsent
messages were not allowed. This change was made because otherwise
the edits were discarded due to use of txnid rather than mxid in the
"m.relates_to" object. Remove this restriction and fix the issue by
replacing txnid with mxid in all related events when the message is
sent (and we obtain mxid from the server).