Remove obsolete admin related to sender verification

(Also match admin view method signatures to base classes and remove
unused imports while we're in there.)

Cherry-picked from medmunds/Djrill@ce69fd78

djrill/forms.py

@@ -1,7 +0,0 @@
-from django import forms
-from django.conf import settings
-from django.core.exceptions import ImproperlyConfigured
-
-
-class CreateSenderForm(forms.Form):
-    email = forms.EmailField()