fix git_export tests
Merge pull request #6238 from edx/clytwynec/fix_failing_git_export_tests

fix git_export tests
Make a generic django field for storing OpaqueKey classes
Extract common base functionality for storing xblock fields in the relation db
Move lms.lib.xblock into lms.djangoapps.lms_xblock in preparation add database backed configuration to it
Add a configuration model for managing XBlockAsides
Teach LMS how to render XBlockAsides

[PLAT-217]
Make ErrorDescriptor more resistent to errors
Merge pull request #6162 from cpennington/xblock-asides

XBlockAsides in LMS
change the context_processor in the shopping cart to return a pointer to a function so that only the templates that actually need to know if the shopping cart button should be shown will actually make the database roundtrips
Merge pull request #6242 from edx/cdodge/move-shoppingcart-button-detection

change the context_processor in the shopping cart to return a pointer to...
Export modulestore-stored asset metadata as XML to exported course.
Import asset metadata XML into modulestore.
Return empty values and log a warning instead of raising NotImplemented
exceptions for XML modulestore asset metadata methods.
Optimize importing many items of asset metadata by avoiding multiple
round-trips to MongoDB.
Fixups resulting from code reviews.