There're some issues with mdVirtualRepeat that required some changes to
our app:
1. Having multiple directives with md-virtual-repeat doesn't work well.
That why I added a dedicated div for the md-virtual-repeat directive.
2. ui-sref-active doesn't work well for child nodes of the
md-virtual-repeat element. The class was applied repetitively to a
visible child. I now save which message or card is selected and use the
ng-class directive to highlight the selected list item.
