xml_importer now calls store.has_course even if create_course_if_not_present is False.

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

TE-489

Remove post-publish step.

STUD-327

LMS-2876

Refactor get_parent_locations STUD-1663

[LMS-2757]

add target_location_namespace check

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]

STUD-1433

Also fixes STUD-1351

STUD-149

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

This reverts commit a517dc20.

This reverts commit ec01253e, reversing
changes made to ae8c9710.

as a first step to separating db connection from modeling
and to support location mapper and contentstore using same db config as modulestore

collection names