Fixes #133

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 one is a bit of a doozy. A summary of the changes:

- Session has grown storage for attachments which have been uploaded but
  not yet sent.
- The list of attachments on a message is refcounted so that we can
  clean up the temporary files only after it's done with - i.e. after
  copying to Sent and after all of the SMTP attempts are done.
- Abandoned attachments are cleared out on process shutdown.

Future work:
- Add a limit to the maximum number of pending attachments the user can
  have in the session.
- Periodically clean out abandoned attachments?

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

The information returned by Client.Mailbox may be out-of-date.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

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

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

Only inline forwarding is supported for now.

References: https://todo.sr.ht/~sircmpwn/koushin/37

This reverts commit 02faf6174b.

This commit doesn't work, because it's applied to <img> tags. Making it
handle <a> tags doesn't work either because bluemonday will strip any
target="_blank" attributes, making the compose form open in the
<iframe>. Let's just revert this whole commit for now.

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

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

Note that attachments will be lost. This is a TODO.

And mark original message as answered.

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