Authors:
  - Daniel Friedman
  - Ben McMorran
  - Peter Fogg

TNL-1892

This commit implements STUD-1490, allowing creation of components
on the container page. It also enables the delete and duplicate
buttons now that new content can be created that would benefit.

Note that it also creates shared functionality for adding components,
and refactors the unit page to use it too.

Added sorting to the new pagination logic for STUD-995.

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

STUD-945

STUD-946
STUD-947

html page and update access use 2 different urls
GET update can get an individual update
STUD-944