Fixes setting acces level to "user"

This wasn't working because "user" isn't a group, it's the absense of
any group.

bookwyrm/forms/__init__.py

@@ -10,3 +10,4 @@ from .landing import *
 from .links import *
 from .lists import *
 from .status import *
+from .user_admin import *

bookwyrm/forms/groups.py

@@ -4,12 +4,6 @@ from .custom_form import CustomForm
 
 
 # pylint: disable=missing-class-docstring
-class UserGroupForm(CustomForm):
-    class Meta:
-        model = models.User
-        fields = ["groups"]
-
-
 class GroupForm(CustomForm):
     class Meta:
         model = models.Group

bookwyrm/forms/user_admin.py

@@ -0,0 +1,10 @@
+""" using django model forms """
+from bookwyrm import models
+from .custom_form import CustomForm
+
+
+# pylint: disable=missing-class-docstring
+class UserGroupForm(CustomForm):
+    class Meta:
+        model = models.User
+        fields = ["groups"]

bookwyrm/views/admin/user_admin.py

@@ -77,6 +77,11 @@ class UserAdmin(View):
     def post(self, request, user):
         """update user group"""
         user = get_object_or_404(models.User, id=user)
+
+        if request.POST.get("groups") == "":
+            user.groups.set([])
+            form = forms.UserGroupForm(instance=user)
+        else:
             form = forms.UserGroupForm(request.POST, instance=user)
             if form.is_valid():
                 form.save()