Persist Mailchimp list data in collection to avoid slow page loads after cache clear

Rebased patch on to the latest 2.x-dev and posted it as a MR. Setting to Needs Review so we get some test feedback.

I did an iteration on the patch. Made some small changes:

• Update documentation to replace mentions of cached lists with stored lists.
• Remove some variable assignments which are no longer needed.
• Align code with latest changes in 2.x-dev branch.

I pushed a temporary second MR that includes the automated test fixed from 📌 2.x branch tests failing Needs review so we can see if we need to update any test coverage.

pfrenssen → closed merge request !64

Used the current MR state with current dev branch on a D10.1.7 and confirm it works.