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
Allow verified to be offered without audit
fixup! Allow verified to be offered without audit
Add the ability to override non-standard verified mode names
Add an optional description field to course modes.
Fix up tests and enrollment behavior.
Course Instructor as financeadmin role, will be able to download all purchase transactions in the course as a CSV file
Move a few HTML elements on the instructor dashboard page
updated the course registration models
added registration-codes generation functionality

rebased and resolve conficts with cdoge/registration_codes

feature enhancement request: added transaction group name text field to the download buttons as an extra optional query paramerter
added django admin pages for Coupon Table

added the suggested changes by cdonge
registration code checkout flow

Redirecting to dashboard after free user enrollment.