fix: Don't generate Config sync messages for unconfigured accounts

Probably sync messages generated for a not yet configured account are useless because there are no
other devices yet. And even if this is not true, we don't want to depend on the order of setting
`Config::SyncMsgs` and other keys. Also w/o this the Python tests don't work if we start syncing
`Config::MvboxMove` because they don't expect that sync messages are sent while configuring
accounts.

src/config.rs

@@ -608,7 +608,7 @@ impl Context {
         mut value: Option<&str>,
     ) -> Result<()> {
         Self::check_config(key, value)?;
-        let sync = sync == Sync && key.is_synced();
+        let sync = sync == Sync && key.is_synced() && self.is_configured().await?;
         let better_value;
 
         match key {