Support Lemonade extensions for attachment forwarding without download on IMAP

Expected behaviour

If an email message in a K9 IMAP account contains attachments, a user should be able to forward the email and its attachments without downloading the attachments.
See also https://code.google.com/archive/p/k9mail/issues/216

Actual behaviour

The user has two options: forward the email without attachments; or download the attachments before forwarding the email.

Specifically, this requires the implementation of three Lemonade extensions: CATENATE, URLAUTH and BURL.

I believe that Isode's and Oracle's email servers already support the server side of these. Dovecot supports two and is scheduled to support the final one in its next release (2.3; https://www.ietf.org/mail-archive/web/apps-discuss/current/msg13450.html ), which should make this far more ubiquitous.

Trojita (and therefore Ubuntu Touch's email client) supports this feature:
https://github.com/benkaiser/trojita/blob/master/docs/masters/mobile-imap.tex

Other useful background on this can be found in the Mail in a Box issue about this:
https://github.com/mail-in-a-box/mailinabox/issues/801

Start by reviewing the CATENATE, URLAUTH, and BURL Lemonade extensions and the linked K-9 Mail, Trojita, and Mail-in-a-Box references. Done means an email in an IMAP account can be forwarded with its attachments without downloading them first, while preserving the existing option to forward without attachments.