Add 'startAtBottom' flag

Controls whether a scrollPanel starts off at the bottom.

This may not be necessary and could either be derived from stickyBottom, but
this means I can be sure that the behaviour of ScrollPanel is completely
unchanged for all other uses to avoid breaking any other uses of
ScrollPanel.

src/components/structures/ScrollPanel.js

@@ -77,6 +77,11 @@ module.exports = React.createClass({
          */
         stickyBottom: React.PropTypes.bool,
 
+        /* startAtBottom: if set to true, the view is assumed to start
+         * scrolled to the bottom.
+         */
+        startAtBottom: React.PropTypes.bool,
+
         /* onFillRequest(backwards): a callback which is called on scroll when
          * the user nears the start (backwards = true) or end (backwards =
          * false) of the list.
@@ -113,6 +118,7 @@ module.exports = React.createClass({
     getDefaultProps: function() {
         return {
             stickyBottom: true,
+            startAtBottom: true,
             onFillRequest: function(backwards) { return q(false); },
             onScroll: function() {},
         };
@@ -324,7 +330,7 @@ module.exports = React.createClass({
      * child list.)
      */
     resetScrollState: function() {
-        this.scrollState = {stuckAtBottom: true};
+        this.scrollState = {stuckAtBottom: this.props.startAtBottom};
     },
 
     /**