Make the initialSyncLimit configurable

I keep wanting to change this to test things, and can never find the right
place to change it. Make it configurable.

src/components/structures/MatrixChat.js

@@ -596,7 +596,7 @@ module.exports = React.createClass({
         Presence.start();
         cli.startClient({
             pendingEventOrdering: "end",
-            initialSyncLimit: 20,
+            initialSyncLimit: this.props.config.sync_timeline_limit || 20,
         });
     },