Moving work on BLD-465 from PR 1811.
Fixing missing import clause in Python.
Addressing DB's comment.

BLD-465.

This applies to global staff (is_staff=True), not course staff.
Previously, staff were granted the Moderator role but not the Student
role upon enrolling in a course. If the Moderator role were later
revoked, then the user would have no role and be unable to post in the
forums, which is confusing for the user. edX staff indicated they would
prefer to not automatically receive the Moderator role, so the Student
role is granted instead. Note that staff will still be able to grant
themselves Moderator privileges through the instructor dashboard if
they wish.

JIRA: FOR-338

Co-author: Alexander Kryklia <kryklia@edx.org>
Co-author: Ned Batchelder <ned@edx.org>
Co-author: Oleg Marchev <oleg@edx.org>
Co-author: Valera Rozuvan <valera@edx.org>
Co-author: polesye
[BLD-384]

The show/hide toggle now properly acts like a button, and focus is now
trapped on the loading element for both the show/hide action and loading
of a new page.

All the dialogs in the wiki are now accessible to screen readers.

Also, the "Switch to selected version" button did the odd thing of
reloading the page with the new version selected, but inside a dialog.
That is now fixed.

The merge dialog has a warning that looks kind of funky still, but you
can read it, unlike the previous code.

LMS-1337

STUD-844

BLD-463

Add release notes for 061a46be, concerning change to beta-tester groups

Now the interface will reset apropriately and allow a user to retry
expanding the discussion or loading a new page, and the alert message
will ask the user to retry.

JIRA: FOR-300, FOR-301

Having ENABLE_S3_GRADE_DOWNLOADS enabled by default in common.py
could lead to surprising behavior for folks downstream. They'd
suddenly see a grade download screen on their new instructor
dashboard, but the links by default would be local files and
couldn't be used in an actual production environment. So we disable
by default and let people explicitly enable it and set it up for
S3 if they wish.

LMS-58

Updated Change Log.

BLD-488

For accessibility purposes, it is bad to allow a user to initiate
loading of additional threads in the navigation sidebar and then shift
focus away from the sidebar, only to have focus snap back when the
additional threads are loaded. Now, we trap focus on the loading element
as recommended by our accessibility consultant.

JIRA: FOR-238

When a user attempts to load more threads in the forum navigation
sidebar, reset the state of the world so the user can retry, and alert
the user appropriately.

AJAX requests on forums pages occasionally fail (usually when a request
to the comments service times out), but the user is not made aware of
the failure. This manifests as either the system not appearing to store
state (e.g. if an attempt to flag a post fails) or something taking
forever to load. Now, a modal will pop up to alert the user that a
request has failed and instruct them to reload the page.

In the longer term, we should fix each user gesture that results in an
AJAX call to gracefully handle a failure by resetting the state of the
world appropriately and aleritng the user.

JIRA: FOR-37

BLD-426

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.

This change requires cs_comments_service version 31ef160 or later. Now
that the /threads endpoint can filter by commentable_ids, use that
instead of the /search/threads endpoint, which does not sort and
paginate correctly.

Bug: FOR-224

They now include screen reader-friendly text, have the ARIA checkbox
role, activate on keypress (for space or enter), and occur in the DOM
after the header information (title, etc.)

Fix bug when the user clicks on the video time-line either
before or after the specified end time, it jumps
to the very beginning of the video.

BLD-392

STUD-846