dovecot: changes to config aligning to 2.4

data/conf/dovecot/conf.d/90-dict.conf

@@ -1,12 +1,12 @@
 # /etc/dovecot/conf.d/90-dict.conf
 # Dict declarations and SQL bindings.
 dict_server {
-  dict sieveafter {
+  dict sieve_after {
     driver = sql
     !include /etc/dovecot/sql/dovecot-dict-sql-sieve_after.conf
   }
 
-  dict sievebefore {
+  dict sieve_before {
     driver = sql
     !include /etc/dovecot/sql/dovecot-dict-sql-sieve_before.conf
   }