Revert stage 2 and 3 of SiteConfiguration/History field rename

[Agrendalath]

Context

The SiteConfiguration and SiteConfigurationHistory models had their values field renamed to site_values in a few PRs from #22692 to #22753. The testing plan has been defined in Data Engineering docs, however these docs don't mention that each stage should be performed in separate release to avoid downtime or altering business logic between releases, as defined in Architecture and Engineering docs.

This is a PoC PR that reverts stage 2 and 3 of #22753. The only conflict was in the common/djangoapps/util/tests/test_db.py file, so these reverts do not seem to be too invasive code-wise. Not sure about any other implications, though. It would be good to get some edX feedback here.

Note: This is missing the relevant migrations, so it won't work as-is. We would need to add back the reverted migrations and then create new migrations that would:

1. Bring back the removed values fields of siteconfiguration and siteconfigurationhistory.
2. Copy the contents of site_values fields into values fields.

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[edx-status-bot]

Your PR has finished running tests. The following contexts failed:

• jenkins/a11y
• jenkins/quality

[natabene]

@Agrendalath Thank you for your contribution. Please let me know once it is ready for our review.

[nedbat]

For context, here is a discussion in Slack about the friction in the Juniper upgrade that this PR is trying to fix: https://openedx.slack.com/archives/CSAC4QHC7/p1591672127105600

[nedbat]

Are we still pursuing this?

[pwnage101]

Based on the conversation in slack that Ned posted, I assume the intention of this PR is to un-do this column rename which currently breaks zero-downtime open edX releases. Since this is not the only column rename, and this also doesn't completely revert the changes for this particular rename (i.e. stage 1 would also need to be reverted for this to make any sense), I would reject it.

[Agrendalath]

@nedbat, @pwnage101, thank you for providing the context here. We're closing this, as these breaking changes were done many months ago and reverting them just for Juniper in a forward-compatible way is tricky due to a few previous reverts, plus Juniper is already released, so many could have already upgraded to it.

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