Make XBlock client-side runtimes proper classes, so that handlerUrl can
be defined in a per-runtime way, and we can have multiple runtimes on a
single page.
[LMS-1630][LMS-1421][LMS-1517]
These changes implement STUD-813. The commit consists of the
following logical changes:
- a REST API has been implemented for a course's assets
- the page itself now fetches the assets client-side
- the Backbone.Paginator library is used to support pagination
- the AssetCollection has been refactored to extend
Backbone.Paginator.requestPager so that it can be paged
- an abstract PagingView class has been added to generalize
the communication with a paging REST API
- the AssetsView has been reimplemented to extend PagingView
- two new child views have been added:
- PagingHeader: the paging controls above the list of assets
- PagingFooter: the paging controls below the assets
