Annotation Tool: Flagging plugin should now allow instructors to remove flags from annotations

Annotation Tools: Added lots of comments, fixed spacing, fixed slice issue

Annotation Tools: i18n fixed for this tool

 - fixed Translator to Translators

Annotation Tools: Removed unnecessary class and worked with element object
Instructors can view previously sent email content

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.
Merge pull request #4378 from lduarte1991/lduarte-harvardx-pr10

Annotation Tool: Fixed Flagging for Instructor flag removal
Merge pull request #4451 from Stanford-Online/njdupoux/email-content-history

Instructors can view previously sent email content
Refactor ADVANCED_COMPONENT_TYPES

    Modified to a setting in CMS common.py and aws.py
    Also factored out are: ADVANCED_PROBLEM_TYPES

    This enables third parties to add XBlocks to their system
    without having to make a code change.

    Code also added to ensure that a component should exist in both
    ADVANCED_COMPONENT_TYPES and the course advanced module list
    in order for it to be enabled in the course.
Added clarification for events that do/not include session values
Merge pull request #4683 from edx/ahodges/documentation/AN2976

Added clarification for events that do/not include session values
Merge pull request #4512 from Stanford-Online/dcadams/refactor_advanced_component_types

Refactor ADVANCED_COMPONENT_TYPES
Fix filter with new opaque key, allowing user to set COURSE_LISTING as old way path
Cleaned pylint quality issues in instructor email content history code.
Merge pull request #4688 from Stanford-Online/njdupoux/email-content-history-qualityfixes

Cleaned pylint quality issues in instructor email content history code
Fix Video Player Initialisation Issue
Merge pull request #4691 from edx/ammar/fix-video-init-issue

Fix Video Player Initialization Issue
Merge pull request #4254 from marcore/fix-branding-course-listing-opaque

Fix filter with new opaque key, allowing user to set COURSE_LISTING as o...
Annotations Tools: Fixing major Firefox/Tinymce bug in annotations

 - fixed issue in Firefox where the init gets called multiple times
Fixed spacing issues and added more comments