This addresses an oversight from when the fifth column (flags) was
introduced.

This was causing a nil pointer dereference before.

Fixes ~migadu/alps#121.

This can happen if an email is deleted by an external client, and this
change fixes a template crash in such cases.

I elected to display (deleted email) rather than to hide deleted
messages so that, if the message is unread, it provides an explanation
for your inbox showing $n unread emails where $n > number of non-deleted
emails.

This partially addresses ~migadu/alps#121

P.a. to 50e131244a which solved
https://todo.sr.ht/~emersion/alps/100

Closes: https://todo.sr.ht/~emersion/alps/102

P.a. to 50e131244a which solved
https://todo.sr.ht/~emersion/alps/100

Closes: https://todo.sr.ht/~emersion/alps/102

This would cause this error:

    template: head.html:7:15: executing "head.html" at <index .Global.Path 0>: error calling index: index of untyped nil

Closes: https://todo.sr.ht/~emersion/alps/100

This is a hacky workaround to make things work until we find a better
solution.

Closes: https://todo.sr.ht/~emersion/alps/96

The information returned by Client.Mailbox may be out-of-date. Instead,
retrieve the mailbox status from the server each time.

Closes: https://todo.sr.ht/~emersion/alps/80

Closes: https://todo.sr.ht/~emersion/alps/51

Fixes panic if there is no usable Sent folder.

Signed-off-by: Wagner Riffel <wgrriffel@gmail.com>