This PR removes prepareMsg, which put messages into the OUT_PREPARING state and was used while videos were recoded. It also removes the now-unused method isIncreation().
- It was buggy because when you forwarded a message while it was InPreparation, or when the app was killed while a message is InPreparation, the message would stay InPreparation forever.
- Android is the only UI using this InPreparation (according to @r10s, I didn't check this myself), so we can simplify some things in core, which will also make it easier to deduplicate blob files.
* Don't always show the sender in notifications
In 1:1 chats, don't prepend the sender name to every line. The exception is when the display name is not equal to the chat name (i.e. there is an OverwriteSenderDisplayname), in this case we still prepend it.
* Always show the "~" before the sender name if it's overridden
When we introduced this, I assume that we weren't sure whether we should
do it and only showed it in some places. But I think it's nicer to
show the same sender name everywhere, i.e. always add the "~".
