add rake task to migrate existing data to commits
add rake task to migrate existing data to commits

This rake task will migrate the existing data to commit-objects and link
ontology_versions appropriately. If we were however to remove the
commit_oid field from ontology_versions we could only do so after this
rake task has been run.

ontohub/ontohub,ontohub/ontohub,ontohub/ontohub,ontohub/ontohub,ontohub/ontohub,ontohub/ontohub