This patch:

1. Copies unsent messages to the outbox before attempting to deliver
   them with SMTP
2. Deletes those messages once they're sent, or leaves them if an error
   occured
3. Updates the message list to make it obvious when there are unsent
   messages in the outbox

This was causing a nil pointer dereference before.

Fixes ~migadu/alps#121.

The information returned by Client.Mailbox may be out-of-date.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

Fixes panic if there is no usable Sent folder.

This allows us to extend these and expose helpers for templates and
plugins.

This alows to add/remove flags instead of setting them.

It's too slow on servers where indexes aren't enabled.

References: https://todo.sr.ht/~sircmpwn/koushin/23

Add a settings page where the user can change the number of messages
displayed per page.

References: https://todo.sr.ht/~sircmpwn/koushin/16

And mark original message as answered.

Closes: https://todo.sr.ht/~sircmpwn/koushin/15

This only reloads templates for now. In the future it'll also reload
Lua plugins.

References: https://todo.sr.ht/~sircmpwn/koushin/36

This is a public type other plugins and templates may want to access.

Closes: https://todo.sr.ht/~sircmpwn/koushin/23

This plugin offers base IMAP/SMTP functionality.

References: https://todo.sr.ht/~sircmpwn/koushin/39

The former fixes encoding issues in filenames. The latter is just for
brevity.

This allows to go back and forth between the mailbox view and the
message view.

References: https://todo.sr.ht/~sircmpwn/koushin/22

References: https://todo.sr.ht/~sircmpwn/koushin/22

This fixes the second part of #14.

References: https://todo.sr.ht/~sircmpwn/koushin/14

This adds support for additional charsets when parsing e.g. subject
lines.

Enables support for non-UTF8 charsets.

Closes: https://todo.sr.ht/~sircmpwn/koushin/18