Add a new boolean radios fields and change forms to use it

This adds a new OnOffField class that implements a boolean field
that is rendered as two On / Off radio buttons. This allows us to
avoid comparing 'on' and 'off' string values in the views since
the field takes care of transforming form data into python booleans.

This also adds a form class that can be used for any single On / Off
switch forms (e.g. service permissions).

app/main/forms.py

@@ -696,23 +696,37 @@ class ServiceLetterContactBlockForm(StripWhitespaceForm):
             )
 
 
-class ServiceSwitchChannelForm(StripWhitespaceForm):
+class OnOffField(RadioField):
+    def __init__(self, label, *args, **kwargs):
+        super().__init__(label, choices=[
+            (True, 'On'),
+            (False, 'Off'),
+        ], *args, **kwargs)
 
-    def __init__(self, channel, *args, **kwargs):
+    def process_formdata(self, valuelist):
+        if valuelist:
+            value = valuelist[0]
+            self.data = (value == 'True') if value in ['True', 'False'] else value
+
+
+class ServiceOnOffSettingForm(StripWhitespaceForm):
+
+    def __init__(self, name, *args, **kwargs):
         super().__init__(*args, **kwargs)
-        self.enabled.label.text = 'Send {}'.format({
+        self.enabled.label.text = name
+
+    enabled = OnOffField('Choices')
+
+
+class ServiceSwitchChannelForm(ServiceOnOffSettingForm):
+    def __init__(self, channel, *args, **kwargs):
+        name = 'Send {}'.format({
             'email': 'emails',
             'sms': 'text messages',
             'letter': 'letters',
         }.get(channel))
 
-    enabled = RadioField(
-        'Choices',
-        choices=[
-            ('on', 'On'),
-            ('off', 'Off'),
-        ],
-    )
+        super().__init__(name, *args, **kwargs)
 
 
 class FieldWithNoneOption():