Extends the common capa response types (string, numeric, multiple
choice, checkbox, dropdown) with feedback and hint
capabilities. "Feedback" refers to feedback shown to the student when
they check the problem, looking at their specific answer. "Hints"
refers to a Hint button in LMS which the student can click at any time
to see hints for that problem. The implementation extends the markdown
syntax to include feedback and hints. There are new Feedback-and-Hint
specific templates in Studio when the author clicks to add a new
problem.

TNL-653

TNL-651

Co-Authored-By: Jean-Michel Claus <jmc@edx.org>
Co-Authored-By: Brian Talbot <btalbot@edx.org>
Co-Authored-By: Tim Babych <tim@edx.org>
Co-Authored-By: Oleg Marshev <oleg@edx.org>
Co-Authored-By: Chris Rodriguez <crodriguez@edx.org>

TNL-735

TNL-670

JIRA: TNL-710

IMPORTANT: this commit converts the course_groups
package to using migrations.  When deploying to an
existing openedx instance, migration 0001 may fail
with an error indicating that the CourseUserGroup
table already exists.  If this happens, running
the 0001 migration first, with the --fake option,
is recommended.  After performing this step,
remaining migrations should work as expected.

TNL-649

TNL-683

Users may want to start anew when answering a question. This commit decouples reset from randomization while still preserving backward compatibility. Users can clear their input.
Instructors can set course-wide and problem-specific settings for reset button display.

TNL-162

Note that the features in this release are opt-in, and course and video
behavior will remain the same unless a course explicitly opts in.

Major pieces of functionality with this commit:

Allows the listing of a user's enrollments, course videos, and updates. In
order to make a course available for mobile use, course staff must explicitly
set the Course Advanced Setting "Mobile Course Available" to true. Course staff
will always see their own courses through the Mobile API regardless of this
setting, but students will only be allowed to see a course through the Mobile
API if this setting is set to "true". By default, a Course will *not* be
available for mobile use.

This is a Django app for video resource management. It is completely optional,
and is intended to allow video and operations teams to create new encodings of
videos (e.g. low res for mobile) and change CDNs without having to edit course
data directly. Course teams can now use a "EdX Video ID" setting for Videos,
which will leverage VAL. Video units that do not fill in an "EdX Video ID" will
behave exactly as they always have.

* The Mobile API is enabled with the ENABLE_MOBILE_REST_API feature flag.
* VAL is enabled with the ENABLE_VIDEO_ABSTRACTION_LAYER_API feature flag.
* VAL and the Mobile API both require ENABLE_OAUTH2_PROVIDER).
* The Mobile API is a read-only API, but VAL requires database migrations.
* Applications that make use of either the Mobile API or VAL must be registered
  with the OAuth2 provider app in Django Admin.

BLD-1207

STUD-1726

Previously on the send email page of the instructor dashboard, instructors could only view
task information about emails they've sent for their course in the past.
In addition to this, I've now added the ability to see the content of all previously sent emails.

A "Sent Email History" button has been added to the page. When clicked, a table displaying the
subject line, number of emails sent, and date/time of submission for each previously sent email
is created. An instructor can then click on any subject line to see the content of that email,
displayed in a modal window that appears on the page.

The window is also equipped with a "copy email to editor" button, which copies the emails contents
to the tinyMCE editor, so that an instructor can easily resend an email that they've sent
in the past.

 Added a new advanced setting; invitation_only

 If invitation_only display "Enrollment in this course is by invitation only"
 Otherwise, if outside of enrollment period display "Enrollment is Closed"

 This issue is logged at LMS-2670

 Also fixed in this PR is issue LMS-2717; even though the Register button was greyed out,
 clicking it sent one to the registration page.

STUD-302, STUD-303

STUD-1658

STUD-1312

refactor _generate_items_for_subtask not to chunk queries

use django's iterator methor for querysets

remove bulk email query settings

use read_replica if available, since this is read-only

update changelog

Changes requested in PR

This commit implements STUD-1490, allowing creation of components
on the container page. It also enables the delete and duplicate
buttons now that new content can be created that would benefit.

Note that it also creates shared functionality for adding components,
and refactors the unit page to use it too.