MELS: check scroll on load + use mels-1,-2,... key

To fix https://github.com/vector-im/riot-web/issues/2916, force the checking of scroll position by calling _onWidgetLoad (might need renaming...) when a MELS is expanded/contracted.

Also use an keying scheme for MELS that doesn't depend on the events contained, but rather a simple incrementing index based on the order of the MELS as it appears amongst all MELS.

src/components/views/elements/MemberEventListSummary.js

@@ -30,6 +30,8 @@ module.exports = React.createClass({
         avatarsMaxLength: React.PropTypes.number,
         // The minimum number of events needed to trigger summarisation
         threshold: React.PropTypes.number,
+        // Called when the MELS expansion is toggled
+        onToggle: React.PropTypes.func,
     },
 
     getInitialState: function() {
@@ -63,6 +65,7 @@ module.exports = React.createClass({
         this.setState({
             expanded: !this.state.expanded,
         });
+        this.props.onToggle();
     },
 
     /**