562978990a
* 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.