* Updates `ModulestoreBlockMigration` table to allow storing `null` values in `target` field for blocks that failed to migrate/import.
* Adds `unsupported_reason` field to store reason for failure.
* Add number of children blocks in failed block `unsupported_reason` field.
* Fixes issue with blocks like `openassessment` where `url_name` field is not included in its olx during serialization.
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