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.

This doesn't work the other way round though because `APPEND` doesn't
have an origin. However, learning mails as spam happens more often than
learning spam as ham, so this is IMHO still useful.

[1] https://sourceforge.net/p/isync/mailman/isync-devel/thread/87y2p1tihz.fsf%40ericabrahamsen.net/#msg37030483

mail-server/dovecot.nix

@@ -243,7 +243,7 @@ in
 
           # From elsewhere to Spam folder
           imapsieve_mailbox1_name = ${junkMailboxName}
-          imapsieve_mailbox1_causes = COPY
+          imapsieve_mailbox1_causes = COPY,APPEND
           imapsieve_mailbox1_before = file:${stateDir}/imap_sieve/report-spam.sieve
 
           # From Spam folder to elsewhere