Migrate channel databases before import to guarantee compatible schema.

[rtibbles]

Summary

Description of the change(s) you made

Adds a tweak to the management command for importing previously published channels to ensure that the channel database of the public channel is migrated, so that the schema is readable by Django.

Manual verification steps performed

1. Downloaded the channel database for this channel: https://unstable.studio.learningequality.org/en/channels/d0cb2b465843584e9c72969ea5ea5519/
2. Ran through the steps of the management command manually
3. Ensured an error only occurred because the Channel wasn't in the contentcuration tables (the last step in the process)

[jamalex]

Looks good!