* Handle grading errors

 Add import_drafts abstract method in addition to import_children.

Change xml file reading code when importing a course.

Only export draft modules that have changes w.r.t. the published branch
should be exported to the /drafts directory in the course export.

Include tests that mimic behavior in PLAT-452.

* Updated XBlock hash - new version forces exposes force_dirty method on fields
* Includes Scope.children fields into xml_importer._update_module_location so that they are updated as well.
* Tests for mutable fields xml_importer._update_module_location with mutable fields and children

Move entry_point down into XModule/XModuleDescriptor so that 'xmodule.v1' only applies to XModules, not pure XBlocks with the XModuleMixin

When there is multiple units in a particular subsection
then there will be tabs in courseware. When url have
tab number at the end of the url the code was not
considering it and showing the wrong tab. I have changed
such that it will always check the value of position.

TNL-1844

TNL-2008

Update Mobile API to include channel_id for course.

This pull request makes the video player module respect
the video start/stop times specified by the instructor
as a hard start/stops, i.e. the video treats the start
time as the beginning of the video and the stop time as
the end.

Previously, the video module displayed the full
video and displayed a jquery ui slider range representing
the start and end times.

This new functionality invalidates two previous acceptance
tests for the video module:

    - slider visible test
    - finish time video acceptance test.

These tests are removed from the common acceptance tests.

Added setting to enable/disable library indexing + moved is_index_enabled into search indexer classes
Added task and signal to fire library reindex