Allow site builders to add tags to members

Problem/Motivation

MailChimp tags are a useful segmenting tool. Up until recently, site builders often used hidden interest groups for this purpose. There's even a patch to this module that makes it easier to integrate with this feature. However, we discovered a bug that prevented interest groups from working correctly. Furthermore, tags appear to be the more appropriate interface for hidden segmentation as of 2020.

We have a couple of patches that accomplished our goal of being able to tag members from D8. Unfortunately, it required patching both the this module as well as the Mailchimp module. The patches are fragile given all the moving parts of the MailChimp ecosystem at the moment. But we thought it would be helpful to document our efforts here, and possibly begin collaborating on a shared solution. You can see our initial implementation with this commit.