Fix showing edited message for historical messages

[TobiasFella]

Fixes #713

This is mostly just extracting the code from Room::Private::addNewMessageEvents in order to be able to reuse it in Room::Private::addHistoricalMessageEvents. Note that there is one change: the range of events being searched for the replaced event is changed from (events.begin(), it) to (events.begin(), events.end()) - Apparently this is required when handling historical messages; I don't know the exact reason, but don't see how it could cause problems.