Stop adding required attribute to WTForm fields

WTForms now renders the `required` attribute if there is a validator
such as `DataRequired`. This was flagged in an accessibility audit as
being unnecessary since it doesn't conform to the Design System
recommendations, which state that "all form fields are considered
mandatory when navigating a government service unless otherwise denoted
by the word ‘(optional)’."

This uses the approach here wtforms/wtforms#361
to overwrite the `render_field` method.

app/main/forms.py

@@ -555,6 +555,10 @@ def bind_field(self, form, unbound_field, options):
             bound.get_form = weakref.ref(form)  # GC won't collect the form if we don't use a weakref
             return bound
 
+        def render_field(self, field, render_kw):
+            render_kw.setdefault('required', False)
+            return super().render_field(field, render_kw)
+
 
 class StripWhitespaceStringField(GovukTextInputField):
     def __init__(self, label=None, param_extensions=None, **kwargs):