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.

STUD-302, STUD-303

http://bgrins.github.io/codemirror-accessible/)
LMS-1802

BLD-426

Cleaned up files with muliline imports

Cleaned up files that do not use these imports

Misread comment

This seems to fix the sporadic failure (at least on my machine).

It turns out that code mirror things can be edited.  Once the div is clicked, a new text area appears that can be obtained and sent keys

* Missing docstring
* Redefining name from outer scope

manipulation