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.

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>