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-327

STUD-1725

LMS-2876

add target_location_namespace check

This changes logic to allow more missed use cases of course_image to
work properly.  The cases are:

. XML courses with the course_image attribute set
. Mongo courses that are imported without a contentstore
. Mongo courses that have course_image set but don't have a content store

It also exports default images_static_course.jpg to
images/static_course.jpg to handle a use case where a course author
uploaded an image to the default location in studio without using the
studio interface for adding course images, they then export the course, and then
import it without a contentstore

Added tests for importing a pure XBlock

Fix donwload subs for non youtube videos and non-en language - continue.
Add acceptance tests.
Add detetion of assets on request.
Updated docstring.
Add fixes and acceptance tests.
Fix acceptance tests.
Update docsrtings and cleanup code, resful for language_id.
Specify exception type in POST.
Fix url in upload module.
Improve exception handling.
Remove 'en' and catching in editable_metadata.
Move descriptor.get_context test to lms tests.
Add query parameter to translation dispatch.
Response to format parameter of translatin GET request.
Fix Acceprance test: Metadata Editor.
move handlers to proper scores.
Split video player into smaller files.
Add ugettext and fix typoes.
Add changelog.
Support for downloading non-ascii filenames.
Change event binding.
Add content-language to download requests.
Reractor POST handler to not update self.transcripts.

STUD-1433

LMS-1192

STUD-1359

Currently wiki_slug is set to course number. However, since multiple
courses can have the same number this may lead to clashes. So wiki_slug
will be set to org.course.name. To maintain the active wiki_slugs for
xml courses this cannot be changed in the CourseDescriptor.

LMS-2136

Also fixes STUD-1351

update course info module to save content in new 'items' field + update import/export to handle content field other than 'data' or 'metadata'
STUD-154

STUD-149

STUD-1027

[LMS-179]
[LMS-209]
[LMS-1345]

Add tests

STUD-714

STUD-878

Part of STUD-91

Part of STUD-868

These files seem to have been copied over from old XML courses,
and shouldn't be in our repository.