Add migration to add data consent boolean, timestamps

[dsubak]

What are the relevant tickets?

https://github.com/mitodl/hq/issues/13262

Description (What does it do?)

This adds the fields we actually care about adding on the b2b contracts m2m through model, namely a boolean tracking data consent and a timestamp for when it was recorded. It also adds row level modification and and created timestamps (technically unnecessary at the moment, could be useful later if we end up adding more fields).

How can this be tested?

Migration should apply without issue. There's no users of this functionality yet, so that's about all that we should expect!

Additional Context

This is a stacked PR on top of the changes that just add the new through model and perform a table rename. I could squish these into one big migration if we'd like, but I think it's nicer to have that separate since the first one is spookier than this one.

Also worth noting is that we have a temporary default created on timestamp specified in the migration. That's because TimestampedModel adds created_on as a non-nullable field. Perfectly reasonable for a net new model, slightly misleading for models which already have records in the table - the practical outcome of this is that all extant records will have the same datetime set. If people don't like that, let me know!

[github-actions]

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Unexpected changes? Ensure your branch is up-to-date with main (consider rebasing).

[jkachel]

Small change - should be good to go once that's fixed, though.

[jkachel]

LGTM 👍 did test on an instance that didn't have the pt1 migrations applied yet