"Ungroup selected" to remove a title from a series does not work as expected

[lizadaly]

Easiest to explain by reproduction case:

• Have 3 published casebooks in your dashboard
• Go to the user dashboard and select Manage Casebooks
• Add 3 books to a series by grouping them
• Select 1 book, click "Ungroup selected"
• Observe that two books move to ungrouped!

What seems to be happening behind the scenes is that upon clicking "Ungroup selected", the front end sends a POST request to /api/titles/<series-id> with a payload of only the current title, and the serializer happily updates the CommonTitle object to match. This seems to be the effect whether you choose the current title to ungroup or not.

I'm pretty sure this has the effect of always removing all titles from a series, no matter how many are in there.