This provides the name portion of your From header in the compose view.

This is less than ideal, but we still won't end up over-reading because
we're using a LimitedReader here.

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 was causing a nil pointer dereference before.

Fixes ~migadu/alps#121.

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

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The information returned by Client.Mailbox may be out-of-date.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