dovecot: Enable the variables Sieve extension

"In advanced mail filtering rule sets, it is useful to keep state or
configuration details across rules.  This document updates the Sieve
filtering language (RFC 5228) with an extension to support variables.
The extension changes the interpretation of strings, adds an action to
store data in variables, and supplies a new test so that the value of a
string can be examined."[1]

[1] https://datatracker.ietf.org/doc/html/rfc5229/

Fix #368

roles/dovecot/templates/dovecot.conf.j2

@@ -40,7 +40,7 @@ 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_extensions = envelope fileinto imap4flags mailbox regex variables
 	sieve_global_dir = /etc/dovecot/sieve/global/
 	sieve_global_path = /etc/dovecot/sieve/default.sieve
 	sieve_before = /etc/dovecot/sieve/spam-to-folder.sieve