LEARNER-394

Ensure Enterprise-integrated courses require user consent before granting access to Wiki and Discussion forum.

* User-facing links are gated; internal services are not.
* Adds view decorator data_sharing_consent_required
* Renames `get_course_specific_consent_url` to `get_enterprise_consent_url`,
  which now checks `consent_needed_for_course` before returning a consent URL.

ECOM-1734

This commit adds the non-courseware lms/djangoapps and lms/lib.

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]

In the current implementation if the path prefix contains the
wiki_path value it gets deleted. For example if the prefix is
'/courses/edx/math101/2014' and wiki_path is '101/' the final
prefix becomes '/courses/edx/math2014'.

LMS-2556

remove unnecessary code, and clarify what it's doing.