[model] Add a PositionedModel as a base class for sorting

Sorting is done via the pos (integer) field
[model] Topic is now a PositionedModel
[serializer] +PositionedListReads (implicit) with 'addPositions'

The addPositions method can be used to set the pos field of the
PositionedModel via the ordering of the List.
[serializer] Conference.topic: call addPosition @ deserialisation
[serializer] Conference.topic: sort before serialisation
[views] template: Add jQuery UI and knockout-sortable [js]
[js/model] Conference.topic: don't sort, it is server-side sorted
[views] admin/conference: make topics sortable

Done by using knockout-sortable (which  uses jQuery UI)