This allows users to use backtab (Shift+tab) to go back through the
fields in the tutorial, like C-K. This then mimics the other methods in
having a forward and backward variant.
Also documented this in the wizard help paragraph.
Many email providers use the imap sub-domain for imap and the smtp
sub-domain for smtp. FastMail is an example of this[1]. This is a small
quality-of-life improvement which automatically replaces imap.* with
smtp.* when going from the imap screen to the smtp screen in the wizard
[1]: https://www.fastmail.com/help/technical/servernamesandports.html
