Permissions for groups can not be revoked w/o complete reset

Created by: temparus

Original permission of a given group

"permissions": {
        "users": "readwrite",
        "sessions": "readwrite",
        "events": "readwrite",
        "eventsignups": "readwrite"
}

The PATCH request to change a group's permissions with

"permissions": {
        "users": "readwrite",
        "sessions": "read",
        "studydocuments": "readwrite"
}

results in

"permissions": {
        "users": "readwrite",
        "sessions": "read",
        "events": "readwrite",
        "eventsignups": "readwrite",
        "studydocuments": "readwrite"
}

In order to revoke any permissions completely for a given group, first we have to patch the permissions field with null before we can patch the resource with the desired permissions.