Add imap-fast-trash patch.

Add imap-fast-trash patch.

This is based on the patch by Paul Miller.

Modifications are:

* Create a new flag, MUTT_TRASH for imap_make_msg_set(), rather than
  use MUTT_EXPIRED.

* Change imap_make_msg_set(MUTT_TRASH) to only look at
  hdrs[n]->deleted && !hdrs[n]->purge, behaving like MUTT_TAG, rather
  than looking at the HEADER_DATA.

* Reimplement imap_fast_trash() based on imap_copy_message().
  It looks the old version was too, but it lacked handling of TRYCREATE
  and also queued the UID COPY but didn't exec it.  (Presumably this
  happened in the subsequent sync).

* Move the Context magic and mx_is_imap() checks outside of
  imap_fast_trash()