Add govukCheckboxesField

govukCheckboxesField subclasses
SelectMultipleField and overwrites how it renders
HTML to let us use the GOVUK Checkboxes component
while retaining all the functionality of WTForms
fields.

Based on work on github.com/richardjpope/recourse:

https://github.com/richardjpope/recourse/blob/master/recourse/forms.py#L6

app/main/forms.py

@@ -3,7 +3,7 @@ from datetime import datetime, timedelta
 from itertools import chain
 
 import pytz
-from flask import request
+from flask import Markup, render_template, request
 from flask_login import current_user
 from flask_wtf import FlaskForm as Form
 from flask_wtf.file import FileAllowed
@@ -487,12 +487,66 @@ class RegisterUserFromOrgInviteForm(StripWhitespaceForm):
     auth_type = HiddenField('auth_type', validators=[DataRequired()])
 
 
-PermissionsAbstract = type("PermissionsAbstract", (StripWhitespaceForm,), {
-    permission: BooleanField(label) for permission, label in permissions
-})
+# based on work done by @richardjpope: https://github.com/richardjpope/recourse/blob/master/recourse/forms.py#L6
+class govukCheckboxesField(SelectMultipleField):
+
+    def __init__(self, label='', validators=None, param_extensions=None, **kwargs):
+        super(govukCheckboxesField, self).__init__(label, validators, **kwargs)
+        # default choices to a single True Boolean
+        if 'choices' not in kwargs:
+            self.choices = [('y', label)]
+        self.param_extensions = param_extensions
+
+    # self.__call__ renders the HTML for the field by:
+    # 1. delegating to self.meta.render_field which
+    # 2. calls field.widget
+    # this bypasses that by making self.widget a method with the same interface as widget.__call__
+    def widget(self, field, **kwargs):
+        items = []
+
+        # error messages
+        error_message = None
+        if field.errors:
+            error_message = {"text": " ".join(field.errors).strip()}
+
+        # convert options to ones govuk understands
+        for option in field:
+            items.append({
+                "name": option.name,
+                "id": option.id,
+                "text": option.label.text,
+                "value": option.data,
+                "checked": option.checked
+            })
+
+        params = {
+            'idPrefix': field.id,
+            'name':  field.name,
+            'errorMessage': error_message,
+            'items': items
+        }
+
+        # add HTML to group items if more than one
+        if len(items) > 1:
+            params.update({
+                "fieldset": {
+                    "attributes": {"id": field.name},
+                    "legend": {
+                        "text": field.label.text,
+                        "classes": "govuk-fieldset__legend--s"
+                    }
+                }
+            })
+
+        # extend default params with any sent in
+        if self.param_extensions:
+            params.update(self.param_extensions)
+
+        return Markup(
+            render_template('vendor/govuk-frontend/components/checkboxes/template.njk', params=params))
 
 
-class PermissionsForm(PermissionsAbstract):
+class PermissionsForm(StripWhitespaceForm):
     def __init__(self, all_template_folders=None, *args, **kwargs):
         super().__init__(*args, **kwargs)
         self.folder_permissions.choices = []