Don't always paginate when mounting a ScrollPanel

Calling just checkFill on DidMount did not initially set the
scrollTop which meant that one back pagination request is always
performed regardless. This meant we would end up rending the
first batch of events, then paginating and re-rendering again
after the pagination got another batch, causing unnecessary render
churn.

src/components/structures/ScrollPanel.js

@@ -157,7 +157,7 @@ module.exports = React.createClass({
     },
 
     componentDidMount: function() {
-        this.checkFillState();
+        this.checkScroll();
     },
 
     componentDidUpdate: function() {