Re-instate hard-deleted records during snapshot

[joelluijmes]

resolves #2819

Description

Checklist

• I have signed the CLA
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• I have updated the CHANGELOG.md and added information about my change to the "dbt next" section.

[jtcohen6]

This looks great @joelluijmes! Nice work on adding the test for "revived" records. (I also quite like that as a descriptor.)

• Could you add a changelog entry?
• For economy's sake, you could cut the now-redundant filters (where snapshotted_data.dbt_valid_to is null) further down in the snapshot_staging_table macro

[joelluijmes]

Oh wups, didn't even see those filters 😅
Didn't think changelog entry was necessary as this feature kinda should have been present in the original PR. But I'll add it nevertheless 👍

[joelluijmes]

BQ test fails, but doesn't seem this change could have introduced it I'd say?

[jtcohen6]

I think that test failure was a one-off, all is well upon re-running.

Thanks for the contribution @joelluijmes!