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 "~".

Before this PR, the "~" was not shown:

in quotes
when copying messages of multiple senders at once (when copying just one message / messages of just one sender, the sender is not included in the copied text, anyway)
In the "Forwarded by..." text of a forwarded message
in notifications

Context: When the sender of a message is not part of the chat (in mailing lists or because they were sending from an alias), the name is prepended with a "~".

This PR needs to be merged after #3441.

deltachat / deltachat-android

Always show the "~" before the sender name if it's overridden #3442