fix git_export tests
fix pep8 violation
Merge pull request #6239 from edx/clytwynec/fix_pep8_violation

fix pep8 violation
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
fixup! Teach LMS how to render XBlockAsides