UI: Sortable changes

Changes:
- metricCollection.js:
* Added comparator which is now Order.

- jquery.sortable.js:
* Updated to latest version.

- router.js:
* Added AdformNotification to show error when cannot fetch from server.

- metricListView.js:
* Updated Sortable initialization with placeholder etc.
* Updated Sortable.sortupdate method to swap Metric elements in
MetricCollection too.
* Debug: Method MetricListView.addMetric now shows initial item order in
title (it does not update on swap, nor should it!).

- site.css:
* Added basic style for sortable placeholder (.sortable-placeholder)

- metricList.html:
* Debug: Minor button changes.