Limited ciphers to the Mozilla modern set from...

Limited ciphers to the Mozilla modern set from https://mozilla.github.io/server-side-tls/ssl-config-generator/ as requested in issue #1228.

setup/mail-dovecot.sh

@@ -84,7 +84,7 @@ tools/editconf.py /etc/dovecot/conf.d/10-ssl.conf \
 	"ssl_cert=<$STORAGE_ROOT/ssl/ssl_certificate.pem" \
 	"ssl_key=<$STORAGE_ROOT/ssl/ssl_private_key.pem" \
 	"ssl_protocols=!SSLv3 !SSLv2" \
-	"ssl_cipher_list=TLSv1+HIGH !SSLv2 !RC4 !aNULL !eNULL !3DES @STRENGTH"
+	"ssl_cipher_list=ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-SHA384:ECDHE-RSA-AES256-SHA384:ECDHE-ECDSA-AES128-SHA256:ECDHE-RSA-AES128-SHA256"
 
 # Disable in-the-clear IMAP/POP because there is no reason for a user to transmit
 # login credentials outside of an encrypted connection. Only the over-TLS versions