Add pagination to files & uploads page

STUD-813
added pagination to studio assets page
Add pagination to Studio's Files and Uploads page

These changes represent the initial implementation of STUD-813
which adds pagination to the Files and Uploads page.

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

Some areas that need further improvement:
 - the page size is hard-coded to 5 to make it easier to reproduce
 - the 'format=json' option to the REST API is non-standard
 - there are no tests (they will be coming in my next commit)

Note that this commit will not be delivered as is, but is provided
for pre-review purposes.
Address issues in first reviews
Improve UX based on feedback from Frances
Fix unit tests
Add some testing around pagination of JSON requests