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?

For now it can only list events for the current month.

References: https://todo.sr.ht/~sircmpwn/koushin/60

A new minimal CardDAV plugin is introduced. It injects a list of e-mail
addresses in the e-mail compose view.

References: https://todo.sr.ht/~sircmpwn/koushin/7

koushin now takes a list of upstream URLs instead of an IMAP and SMTP
URL. This allows to specify upstream server URLs for plugins. In the
future, this will allow for auto-discovering upstream servers based on a
single domain name.

References: https://todo.sr.ht/~sircmpwn/koushin/49

There's no way around having a global mutex, because we need to update
the HTTP routes when reloading plugins. During reload we need to lock
the whole server.

Closes: https://todo.sr.ht/~sircmpwn/koushin/43

References: https://todo.sr.ht/~sircmpwn/koushin/43

This will be useful to implement hot reload.

This plugin offers base IMAP/SMTP functionality.

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

References: https://todo.sr.ht/~sircmpwn/koushin/1