Import asset metadata XML into modulestore.
Optimize importing many items of asset metadata by avoiding multiple
round-trips to MongoDB.

This commit puts a limit on the size of files that
course staff can upload to MongoDB. The limit is
enforced on the frontend in javascript as well
as backend via the /upload endpoint.

The limit is hard-coded in cms/envs/common.py and
may be changed according to the user's custom
needs.

If the user tries to upload a file that's too
large, an error message will pop up, with a
customizable url that pointing the user to an
external page with an alternate upload procedure.
This url is specified im cms/envs/common.py.
If not set, this url will not be displayed.

AssetKey and the metadata store
PLAT-204

TE-489

First step in moving assets out of the contentstore (optionally).
https://openedx.atlassian.net/browse/PLAT-77

Remove post-publish step.

Refactor get_parent_locations STUD-1663

[LMS-2757]

This commit adds all of cms.

These keys are now objects with a limited interface, and the particular
internal representation is managed by the data storage layer (the
modulestore).

For the LMS, there should be no outward-facing changes to the system.
The keys are, for now, a change to internal representation only. For
Studio, the new serialized form of the keys is used in urls, to allow
for further migration in the future.

Co-Author: Andy Armstrong <andya@edx.org>
Co-Author: Christina Roberts <christina@edx.org>
Co-Author: David Baumgold <db@edx.org>
Co-Author: Diana Huang <dkh@edx.org>
Co-Author: Don Mitchell <dmitchell@edx.org>
Co-Author: Julia Hansbrough <julia@edx.org>
Co-Author: Nimisha Asthagiri <nasthagiri@edx.org>
Co-Author: Sarina Canelake <sarina@edx.org>

[LMS-2370]

- Changed link URL of file name to use public/LMS url
- Added toggle link to display all public URLs for instructor convenience

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

Updated the pagination tests to no longer use the toy course, but to instead explicitly upload the assets it needs.

 Added a new AssetsTestCase base class to own shared helper methods.

STUD-878

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

cleanup

Updates.

Testing changes.

Testing changes.

asset-index progress state cleanjup.

cleanup