use the Dovecot managesieve service to manage sieve scripts

This lets roundcube's manageseive plugin do cool things like vacation responses.

Also:

* Run the spam filtering sieve script out of a global sieve file that we'll place in /etc/dovecot. It is no longer necessary to create per-user sieve files for this. Remove them with a new migration. Remove the code that created them.

* Corrects the spam script. Backslashes were double-escaped probably because this script started embedded within the bash script. Not sure how this was working until now.

this adapts work by @h8h in #103

conf/sieve-spam.txt

@@ -1,7 +1,7 @@
 require ["regex", "fileinto", "imap4flags"];
 
 if allof (header :regex "X-Spam-Status" "^Yes") {
-  setflag "\\\\Seen";
+  setflag "\\Seen";
   fileinto "Spam";
   stop;
 }