Implement direct-to-event linking.

This adds support for links to particular event ids: add /<eventId> to the URL
for a room.

This commit also ensures that we scroll to the 'read marker' when switching to
a room which has no previous scroll state, as well as preventing that marker
from going past the middle of the screen.

This also reinstates the preservation of scroll state when switching rooms,
which was disabled previously.

src/components/views/rooms/EventTile.js

@@ -98,6 +98,9 @@ module.exports = React.createClass({
 
         /* a function to be called when the highlight is clicked */
         onHighlightClick: React.PropTypes.func,
+
+        /* is this the focussed event */
+        selectedEvent: React.PropTypes.bool,
     },
 
     getInitialState: function() {
@@ -273,6 +276,7 @@ module.exports = React.createClass({
             ) !== -1,
             mx_EventTile_notSent: this.props.mxEvent.status == 'not_sent',
             mx_EventTile_highlight: this.shouldHighlight(),
+            mx_EventTile_selected: this.props.selectedEvent,
             mx_EventTile_continuation: this.props.continuation,
             mx_EventTile_last: this.props.last,
             mx_EventTile_contextual: this.props.contextual,