LMS-2194

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.

This code adds the ability to add Mako template lookup directories on
the fly, allowing third party add-ons to contribute their own Mako templates.
A new API function for registering Mako templates is introduced::

    from edxmako import add_lookup

    add_lookup('main', '/path/to/templates')

    # Or, specify a package to lookup using pkg_resources.  This will
    # add the 'templates' directory inside the current package:
    add_lookup('main', 'templates', package=__name__)

The relevant parameters and data are merely passed through between the
front end and comments service.

Part of STUD-1030

The motivation for this change is performance. The forums UI code gets
the list of users for each role and renders the staff label based on
those lists. The list for the staff role is expensive to compute because
there is no index on the is_staff attribute, and we cannot create one
because the User model is built into django.

Users with is_staff=True are still assigned the Moderator role upon
enrolling in a course, so this change will have no practical effect
except that a user who is granted staff privileges after enrolling in a
course will have to be made a Moderator in order for their posts to be
labeled.

Additionally, the UI did not use the list of users with the Student
role, so that list has been removed as well.

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

JIRA: FOR-250

The _DISCUSSIONINFO global was originally used as a cache, but has since
lost that capability and is therefore just harmful. This is a precursor
to more refactoring that will improve the performance of the forums and
may itself provide some performance improvement because it separates the
computation done by two functions that each previously computed the
entirety of _DISCUSSIONINFO.

This is in preparation for significant refactoring of the code in
question.

modification of @jrbl's version to handle a dict with value None

Make django use_tz

This reverts commit 2df3fe93.

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

This reverts commit acfd0d9f, reversing
changes made to e4194c99.

remove discussion caching for now. It might be overkill and I need to figure out how to serialize out the resultset into the cache as writing modules into the cache causes a pickle error

optimize forum page rendering as we don't pre-compute the link urls to inline discussions. We can use jump_to urls and then figure out the link path if/when end-user clicks on it. This saves a lot of unnecessary round trips to the DB as path computation is expensive, especially when it being done for every discussion module in a course in a loop.

This reverts commit e4819a1b, reversing
changes made to b6ec41ca.

Now that we're ready to test again, need to revert the revert to put the filtering UI back in.