Fixup of Bug/djpyfs

Merge pull request #4932 from edx/waheed/lms11266-modify-request-type-for-change-enroll-and-bulk-beta

Modified request method for change enrollment and bulk beta modify access on instructor dashboard.

LMS-11266

add tests for drag and drop unit into collapsed subsection on course out...

Sjang92/advanced settings feedback

Removed grading and drag'n'drop

Bug/djpyfs

This commit adds validation for course advanced settings. Currently when course
administrators make invalid changes in the Settings/Advanced Settings tab,
they're not notified through a new modal window of the list of invalid settings
changes.

* Extending CourseMetadata
    - Previously, we only had update_from_json method in CourseMetadata.py,
      and it was only validating one field every POST request.
    - Now we have validate_and_update_from_json method that encapsulates the
      functionality of update_from_json into a validation call
    - To avoid discrepancy of validation standards between modules, validation
      uses the from_json method implemented to each field in xblock.

* Different Response in advanced settings ajax requests
    - After receiving a POST ajax request, course.py calls
      validate_and_update_from_json, and sends a json object of either:
        1) valid course metadata model
        2) error objects

* Error Messages shown in validation-error-modal
    - error objects passed through ajax are shown in a separate modal.

doc-byte-sized-tasks: Add link to the byte-sized tasks in CONTRIBUTING

Split/lms bugs

Thanks to @adampalay, we now have a list of byte-sized tasks, which are
useful for new contributors who want a gentle introduction to the
platform. This PR adds a link to the tasks tagged as byte-sized in the
public tracker.

Pin coffee-script version to 1.6.1.

1.6.3 is causing some issues.

Update test_group_configuration_validation on single group allowed

Make sure that the currently-set language is the one that shows on the left sidebar if no language  preference exists.

Enable Bok Choy split test.

LMS-11260

to generate url.
LMS-11209

Update opaque-keys library to use block_id for html_id for new keys.

Catch more places where get_children was giving us version aware references.
LMS-11237

one that shows on the left sidebar if no language
preference exists.

ECOM-135

Revert "Update opaque-keys library to use block_id for html_id for new k...

Adds pyfilesystem service from XBlocks to edx-platform

Update opaque-keys library to use block_id for html_id for new keys.

Catch more places where get_children was giving us version aware references.
LMS-11237

Update activation email text

Fix check and show answer for split

Updating the footer for edx.org and open edx.