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
adding sass styles back in for studio assets page
A few more bug fixes