reimplement typing notif timeline shrinking prevention

instead of setting a min-height on the whole timeline,
track how much height we need to add to prevent shrinking
and set paddingBottom on the container element of the timeline.

src/components/structures/TimelinePanel.js

@@ -939,7 +939,7 @@ var TimelinePanel = React.createClass({
             // clear the timeline min-height when
             // (re)loading the timeline
             if (this.refs.messagePanel) {
-                this.refs.messagePanel.clearTimelineHeight();
+                this.refs.messagePanel.onTimelineReset();
             }
             this._reloadEvents();