Change publisher groups from open to restricted (and vice versa)

### Steps to reproduce
Attempt to change a publisher group from open to restricted, or vice versa.
1. While logged in as a Hypothesis admin, go to https://hypothes.is/admin/groups
2. Find a publisher group and go to the admin page, where you would add/remove members (NOT the group activity page)(Hypothesis staff can use our BioPub Editors Restricted group to test with).
3. Attempt to change the "Group Type" value from Open to Restricted, or vice versa, by clicking on the dropdown:

4. Scroll down and click Save.

### Expected behaviour
You should see a flash message across the top of the group admin page saying your changes were successfully saved.

### Actual behaviour
You'll get an error message:

### Browser/system information
I'm using the latest version of Chrome for Mac; Hypothesis team members have reproduced on others systems/browsers

### Additional details
This is a regression from former functionality. We do not currently have any groups that need to have their types changed, but this is something we frequently do for partners as they get to know our product and rethink the way they want to use it.