Set `rfc724_mid` in `Message::new()`, `Message::new_text()`, and
`Message::default()` instead of when sending the message. This way the
rfc724 mid can be read in the draft stage which makes it more consistent
for bots. Tests had to be adjusted to create multiple messages to get
unique mid, otherwise core would not send the messages out.
With iOS and Desktop copying the file to a to a temp file with the name
of `get_filename()`, it should be sanitized first.
The PR can be reviewed commit-by-commit or all at once.
