Use unambiguously magic value for None choices

WTForms coerces `None` as a choice to `'None'` as a string when
rendering form fields (form fields will only ever have string data
because that what the browser posts back).

But internally WTForms coerces `None` to mean an unset value, ie where
the user hasn’t selected a radio button:
283b280320/src/wtforms/utils.py (L1-L20)

We shouldn’t use `None` to mean two different things. And in fact we
can’t, because it in effect means that we’re always getting a value
for the `move_to` field, even if the user hasn’t chosen to move any
templates. Which results in some very expected behaviour.

app/main/forms.py

@@ -707,24 +707,26 @@ class ServicePostageForm(StripWhitespaceForm):
 
 class FieldWithNoneOption():
 
-    # This needs to match the data the browser will post from
-    # <input value='None'>
-    NONE_OPTION_VALUE = 'None'
+    # This is a special value that is specific to our forms. This is
+    # more expicit than casting `None` to a string `'None'` which can
+    # have unexpected edge cases
+    NONE_OPTION_VALUE = '__NONE__'
 
+    # When receiving Python data, eg when instantiating the form object
+    # we want to convert that data to our special value, so that it gets
+    # recognised as being one of the valid choices
+    def process_data(self, value):
+        self.data = self.NONE_OPTION_VALUE if value is None else value
+
+    # After validation we want to convert it back to a Python `None` for
+    # use elsewhere, eg posting to the API
     def post_validate(self, form, validation_stopped):
-        if self.data == self.NONE_OPTION_VALUE:
+        if self.data == self.NONE_OPTION_VALUE and not validation_stopped:
             self.data = None
 
 
 class RadioFieldWithNoneOption(FieldWithNoneOption, RadioField):
-
-    def validate(self, *args, **kwargs):
-
-        if self.data == self.NONE_OPTION_VALUE:
-            self.data = None
-            return True
-
-        return super().validate(*args, **kwargs)
+    pass
 
 
 class HiddenFieldWithNoneOption(FieldWithNoneOption, HiddenField):
@@ -1194,10 +1196,13 @@ class TemplateAndFoldersSelectionForm(Form):
         self.op = None
         self.is_move_op = self.is_add_folder_op = self.is_add_template_op = False
 
+        if current_folder_id is None:
+            current_folder_id = RadioFieldWithNoneOption.NONE_OPTION_VALUE
+
         self.move_to.choices = [
             (item['id'], item['name'])
             for item in ([self.ALL_TEMPLATES_FOLDER] + all_template_folders)
-            if item['id'] != str(current_folder_id)
+            if item['id'] != current_folder_id
         ]
 
         self.add_template_by_template_type.choices = list(filter(None, [
@@ -1237,5 +1242,6 @@ class TemplateAndFoldersSelectionForm(Form):
     move_to_new_folder_name = StringField('Folder name', validators=[required_for_ops('move_to_new_folder')])
 
     add_template_by_template_type = RadioFieldWithNoneOption('Add new', validators=[
+        Optional(),
         required_for_ops('add_template')
     ])