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

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

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

This makes the buttons larger and easier to notice/click.

This also makes the appearance of HTML emails more consistent with
plaintext ones.

This patch contains a whole lot of layout improvements for the alps
theme, mainly replacing the table soup with flexbox and CSS grids, and
fixing up a number of loose ends. This gives us a lot more flexibility
over how the page is laid out. I also cleaned up a lot of other
low-hanging fruit in the layout & styles.