Users should not be able to remove themselves from "Public" group

[mccalluc]

Commit: 33de42c

Steps to reproduce

• Click "Collaboration" at the top
• In the "Groups" pane, click the gear beside "Public"
• Click "Leave"

Observed behavior

Once this is done, the user will no longer have access to public datasets

Expected behavior

Not sure? An error? or no gear beside "Public"?
(Before diving into this, confirm that we're modelling this appropriately... Maybe groups should only be used for restrictions, and a public dataset just has no groups? That might run into other problems, though.)

[hackdna]

data set sharing / collaboration