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?

Fixes some visual weirdness in an edge case

The and/or operators in text/template don't short-circuit, so this
seemingly benign change caused a breakage when not fully tested.

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