Set permissions with checkboxes, not yes/no inputs

The yes/no pattern didn’t work too well, because:
- it didn’t read naturally as a question and answer
- often users left them completely unclicked if they didn’t want to set
  the permission (rather than clicking no)

This commit changes both the invite and edit user pages to use
checkboxes to set permissions. If also rewords these pages to read more
naturally, and explain what the permissions mean.

This meant changing some of the view logic around invites and
persmissions, and I ended up refactoring a bunch of it because I found
it hard to understand what was going on.

app/main/forms.py

@@ -96,31 +96,16 @@ class RegisterUserFromInviteForm(Form):
     email_address = HiddenField('email_address')
 
 
-# WTF forms does not give a handy way to customise error messages for
-# radio button fields so just overriding the default here for use
-# in permissions form.
-class CustomRadioField(RadioField):
+class PermissionsForm(Form):
 
-    def pre_validate(self, form):
-        for v, _ in self.choices:
-            if self.data == v:
-                break
-        else:
-            raise ValueError(self.gettext('Choose yes or no'))
+    send_messages = BooleanField("Send messages from existing templates")
+    manage_service = BooleanField("Modify this service, its team, and its templates")
+    manage_api_keys = BooleanField("Create and revoke API keys")
 
 
-class PermisisonsForm(Form):
+class InviteUserForm(PermissionsForm):
 
-    # TODO fix this Radio field so we are not having to test for yes or no rather
-    # use operator equality.
-    send_messages = CustomRadioField("Send messages", choices=[('yes', 'yes'), ('no', 'no')])
-    manage_service = CustomRadioField("Manage service", choices=[('yes', 'yes'), ('no', 'no')])
-    manage_api_keys = CustomRadioField("Manage API keys", choices=[('yes', 'yes'), ('no', 'no')])
-
-
-class InviteUserForm(PermisisonsForm):
-
-    email_address = email_address('Their email address')
+    email_address = email_address('Email address')
 
     def __init__(self, invalid_email_address, *args, **kwargs):
         super(InviteUserForm, self).__init__(*args, **kwargs)