Use email fields for feedback form

Otherwise we can end up collecting invalid email addresses…

This required some refactoring to allow our email fields to be optional
(but not by default).

app/main/validators.py

@@ -38,6 +38,10 @@ class CsvFileValidator:
 class ValidGovEmail:
 
     def __call__(self, form, field):
+
+        if field.data == '':
+            return
+
         from flask import url_for
         message = (
             'Enter a government email address.'
@@ -53,10 +57,15 @@ class ValidEmail(Email):
         super().__init__('Enter a valid email address')
 
     def __call__(self, form, field):
+
+        if field.data == '':
+            return
+
         try:
             validate_email_address(field.data)
         except InvalidEmailError:
             raise ValidationError(self.message)
+
         return super().__call__(form, field)