FEDX-24

Authors:
  - Daniel Friedman
  - Ben McMorran
  - Peter Fogg

TNL-1892

TNL-2384

Refactored Studio's PagingView to use RequireJS Text and moved it
to common so that it can also be used by LMS.

1. Spliting strings inside the call to `gettext()` in javascript breaks the string extraction process, so the string should kept in one line.
2. Using escape string inside a string in `gettext()` may cause the extractor to misunderstand the escape string, so that it will escape this escape string.

TNL-997

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