[LMS-2757]

Previously, an error was raised if the comments service returned data
including an unexpected field, which unnecessarily complicated the
release path for new features, since the list of allowed fields would
need to be modified before cs_comments_service could be modified, and
only then could edx-platform take advantage of the new CS feature. We
still log a warning if an unexpected field is returned, so we will
still be able to tell if the CS returns a corrupt response.

JIRA: FOR-180

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]

The wrong permission was being checked for pinning/unpinning, so a
sufficiently clever student could pin and unpin threads.

JIRA: FOR-499

Some messages generated by the comments service are not readily
translated because they come from third-party libraries. Thus, we plan
to try to avoid showing any comments service generated message to the
user. This check preempts the only end-user-visible CS-generated error
message that we are presently aware of.

JIRA: FOR-398, FOR-399

This button has been broken for at least a year, so the code has
suffered from bit rot and should be reimplmemented if the feature is
necessary in the short term (which is unlikely since it has been broken
for so long).

Avoid recomputing course module information for every thread, which
should dramatically improve the performance of high-percentile latency
queries.

JIRA: FOR-250

This reverts commit 2df3fe93.

Conflicts:
	common/static/coffee/src/discussion/utils.coffee

This reverts commit acfd0d9f, reversing
changes made to e4194c99.