mail-server/dovecot: also learn spam/ham on APPEND
The current configuration doesn't work when moving spam from the INBOX
to Junk on a local maildir and then syncing the result to the IMAP
server with mbsync(1). This is because mbsync(1) doesn't support a
mvoe-detection[1] (i.e. an IMAP MOVE which subsequently causes a Sieve
COPY according to RFC6851 which then triggers report{h,sp}am.sieve), but
instead sends APPEND (and removes the message in the src mailbox after
that).
Tested on my own mailserver that this fixes spam learning.
cc @lewo