dovecot: Only enable needed sieve extensions

The enotify extension can be used to send mails as other users, so let's
only enabled the sieve extensions, which we need.
The extensions was found by running `grep require` on the existing sieve
user scripts. mailbox wasn't in that list, but it is used by the global
spam-to-folder.sieve script.

roles/dovecot/templates/dovecot.conf.j2

@@ -39,6 +39,8 @@ passdb {
 plugin {
 	sieve = ~/.dovecot.sieve
 	sieve_dir = ~/.sieve
+	# fileinto and mailbox is required by the global spam-to-folder.sieve script
+	sieve_extensions = envelope fileinto imap4flags mailbox regex
 	sieve_global_dir = /etc/dovecot/sieve/global/
 	sieve_global_path = /etc/dovecot/sieve/default.sieve
 	sieve_before = /etc/dovecot/sieve/spam-to-folder.sieve