Add tests of cross-modulestore import/export

[LMS-2945]
Skip DraftVersioningModuleStore tests
Add the manual testing course for testing xml import/export across modulestores
Explicitly create the AssetKey for all assets coming out of a ContentStore
Don't use raw asserts in split (to handle different types of UsageKeys sanely)
Only compute block types with children once
Use field scopes to compute which metadata to strip from the definition during XML import of XMLModule subclasses
Use bok_choy actions rather than actions chains directly to click on css