fix(mail(js)): hide sign and encrypt options if not usable

When composing a message, don't offer to sign nor encrypt the message
if user has not uploaded a certificate or if S/MIME is disabled.

UI/PreferencesUI/UIxJSONPreferences.m

@@ -410,7 +410,7 @@ static SoProduct *preferencesProduct = nil;
         {
           auxAccount = [accounts objectAtIndex: i];
           security = [auxAccount objectForKey: @"security"];
-          if (security)
+          if (security && [domainDefaults mailCertificateEnabled])
             {
               limitedSecurity = [NSMutableDictionary dictionaryWithDictionary: security];
               if ([limitedSecurity objectForKey: @"certificate"])