Adds a new Email link to the instructor dashboard for frontend interface to send
email to course members. Adds a feature flag ENABLE_INSTRUCTOR_EMAIL to toggle this.
Creates a new djangoapp bulk_email that handles this action by getting the recipient
list and batching the emails to different celery tasks to do the actual sending.
Requires lynx package to convert HTML email to plaintext. Handles SMTP errors by
retrying or falling through to the next email. Adds the option to opt out of course
specific emails in the user dashboard with an Email Settings link for each course.
Uses severable configurable settings with defaults. DEFAULT_BULK_FROM_EMAIL
specifies the from address for email. EMAILS_PER_TASK specifies the number of emails
each celery task takes on. EMAIL_HOST, EMAIL_PORT, EMAIL_HOST_USER,
EMAIL_HOST_PASSWORD, and EMAIL_USE_TLS for the SMTP email backend settings.

Co-authored-by: Akshay Jagadeesh <akjags@gmail.com>

Inspired by: http://eldarion.com/blog/2013/02/14/entry-point-hook-django-projects/
Moves startup code to lms.startup and cms.startup, and calls the startup
methods in wsgi.py and manage.py for both projects.

Rather than just calling `complex(...)` on the staff answers, also call
`calc.evaluator`. This way, simple things like `1/3` can be entered, as well
as more complicated explicit equations (e.g. `sqrt((12^2 + 26^2)/2)` etc, etc)

Simplify and update the tests to work with this.

Features coming down the pipe will want to be able to:
* Refer to enrollments before they are actually activated (approval step).
* See what courses a user used to be enrolled in for when they re-enroll in
  the same course, or a different run of that course.
* Have different "modes" of enrolling in a course, representing things like
  honor certificate enrollment, auditing (no certs), etc.

This change adds an is_active flag and mode (with default being "honor").
The commit is only as large as it is because many parts of the codebase were
manipulating enrollments by adding and removing CourseEnrollment objects
directly. It was necessary to create classmethods on CourseEnrollment to
encapsulate this functionality and then port everything over to using them.

The migration to add columns has been tested on a prod replica, and seems to be
fine for running on a live system with single digit millions of rows of
enrollments.

Also fixes a bug where message was not sent to user when entering 'denied'
state unless the user was previously in 'granted'.

Send notification to admin when a user enters 'pending' state.

Pylint cleanup.

STUD-554

Code review feedback.

Refactors trigger function.
Fixes Ui bugs: captions.
Fixes bug with Youtube on page load.
Hotfixes html5 video streaming bug.
Adds test video files to the project.

Add audit log definition, and use for logging of logins in external_auth and student apps.  Move test_login to student app. Improve conditional tests for Shibboleth login logic.  (Does not include reconfiguring log settings.)

Two endpoints are intended to be used by AJAX calls from the web UI with an
authenticated user. When enabled, the value of the preference is an encryption
of the username, to be used as a token for one-click unsubscribe links. The
third endpoint is the target of unsubscribe links, which displays a page with
an appropriate message to the user.

The new API uses Django REST Framework. For now, it is designed specifically
to support the use cases required by the forum digest notifier (not yet built),
with a goal of making it more generally useful over time.

Update existing tests and provide new ones to test new paradigm.

Instead, we use XModule field default values when creating an empty
XModule. Driven by this use case, we also allow for XModules to be
created in memory without being persisted to the database at all. This
necessitates a change to the Modulestore api, replacing clone_item with
create_draft and save_xmodule.

Rather than asynchronously saving when a setting is updated, we now
prompt the user to confirm their changes and only persist the data if
they hit the save button. Lettuce tests are updated to expect this
behavior and some new ones are added.

Optionally email students on enroll/un-enroll actions
by instructor from enrollment tab in LMS.