support previous_vendor_id attribute on System model

[adamsachs]

part of https://ethyca.atlassian.net/browse/PROD-1296

Description Of Changes

Adds a new, optional previous_vendor_id attribute/field to the System model. This field will help us indicate whether a given vendor record (or a system based on that vendor) used to be represented by a different vendor record, e.g. if a google AC vendor transitions to a GVL vendor, as is expected to happen somewhat regularly.

Code Changes

• add previous_vendor_id attribute to System model
• some test cleanup, add a TODO comment since i realized some of our model tests aren't really effective...

Steps to Confirm

• will test out by publishing an alpha tag and using this in Compass webservice code to ensure it can support this new column on Compass data. UPDATE: tested with 2.2.1a0 alpha package, works well 👍

Pre-Merge Checklist

• All CI Pipelines Succeeded
• Documentation Updated
• Issue Requirements are Met
• Relevant Follow-Up Issues Created
• Update CHANGELOG.md

[adamsachs]

this is unrelated to the change in this PR, but saw this test mistake while i was updating the test. i moved this up to the PrivacyDeclaration where it should be, this was just a mindless mistake to have it on the System in the first place.

more importantly, the test has been passing even with this error in here, which means that the test is ineffective! i put in a TODO comment for now, i'll see if i can circle back to improving the test...

[adamsachs]

this doesn't really matter but figured i'd update to make the value more realistic in our test case