Add an option to disable encryption in new chats

Right now NeoChat (or more technically, libQuotient) decides to use
encryption by default in new chats. Some users may not prefer or need
this, so a new option is added under Security to change this behavior.

BUG: 498375

src/neochatconnection.cpp

@@ -145,6 +145,10 @@ void NeoChatConnection::connectSignals()
             });
         },
         Qt::SingleShotConnection);
+    setDirectChatEncryptionDefault(NeoChatConfig::preferUsingEncryption());
+    connect(NeoChatConfig::self(), &NeoChatConfig::PreferUsingEncryptionChanged, this, [] {
+        setDirectChatEncryptionDefault(NeoChatConfig::preferUsingEncryption());
+    });
 }
 
 int NeoChatConnection::badgeNotificationCount() const