Bnei-Baruch/mdb

Implementation notes for the import from kmedia
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Basic Schema Mappings
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Virtual Lesson -> Collection
Container      -> Content Unit
File Asset     -> File


Minimal import unit is a lesson with all its parts and files in. This is a single `import_kmedia` operation.
Each such operation is a single DB transaction.

Issues:
* Collection i18n (see relevant TODO)
* Censor workflow information in container (see relevant TODO)
* Removing stale data from MDB (see relevant TODO)
* file_assets missing SHA1 (Vova)
* SHA1 for files with physical size = 0 (Vova)
* file_assets associated with many containers (m2m)
* containers with virtual_lesson_id = 0 (unassociated data, 16459 containers [5063 lessons], 110299 files [17674 lessons])