Prevent unintentional removal from group for anonymous users filing out profiles

Currently, if an anonymous contact submits a profile that contains a groups field and the profile submission is deduped on submission with an existing contact, the group selections in the profile overwrite the contact's previous groups, for the groups that are included in the profile (i.e. the public groups). The result is that if someone is subscribed to a mailing list and fills in a profile anonymously and doesn't check the same group name, they will be unsubscribed from the group. This is definitely not what users expect! I think it's clear that checking the box will add you to the group, but I doubt many expect that they will be removed from a group if they don't check the box that they've checked in the past.

This is also not what happens with other profile fields that are left blank, nor is it what happens if two contacts are merged manually.

I propose to ignore group checkboxes that are not checked in a profile submitted anonymously. No change for profiles submitted with known cids (as these will have the appropriate checkboxes already checked, allowing the contact to unsubscribe by unchecking them if desired). Currently, all group ids in the profile are being passed here, removing the contact from groups that were unchecked. Will submit PR if supported.

EDIT: Here is some discussion on StackExchange: Unwanted unsubscribe in profile, Anonymous user filing out a profile unintentionally remove themselves from groups