Convert checkboxes for broadcast areas

Includes removal of MultiCheckboxField due to it
no longer being used elsewhere in this file.

app/main/forms.py

@@ -38,7 +38,6 @@ from wtforms import (
 )
 from wtforms.fields.html5 import EmailField, SearchField, TelField
 from wtforms.validators import URL, DataRequired, Length, Optional, Regexp
-from wtforms.widgets import CheckboxInput, ListWidget
 
 from app import format_thousands
 from app.main.validators import (
@@ -118,11 +117,6 @@ def get_next_days_until(until):
     ]
 
 
-class MultiCheckboxField(SelectMultipleField):
-    widget = ListWidget(prefix_label=False)
-    option_widget = CheckboxInput()
-
-
 class RadioField(WTFormsRadioField):
 
     def __init__(
@@ -2135,11 +2129,13 @@ class AcceptAgreementForm(StripWhitespaceForm):
 
 class BroadcastAreaForm(StripWhitespaceForm):
 
-    areas = MultiCheckboxField('Choose areas to broadcast to')
+    areas = govukCheckboxesField('Choose areas to broadcast to')
 
     def __init__(self, choices, *args, **kwargs):
         super().__init__(*args, **kwargs)
         self.areas.choices = choices
+        self.areas.render_as_list = True
+        self.areas.param_extensions = {'fieldset': {'legend': {'classes': 'govuk-visually-hidden'}}}
 
     @classmethod
     def from_library(cls, library):

app/templates/views/broadcast/areas.html

@@ -1,6 +1,5 @@
 {% from "components/page-header.html" import page_header %}
 {% from "components/page-footer.html" import sticky_page_footer %}
-{% from "components/checkbox.html" import checkboxes %}
 {% from "components/form.html" import form_wrapper %}
 {% from "components/live-search.html" import live_search %}
 
@@ -17,10 +16,10 @@
     back_link=url_for('.choose_broadcast_library', service_id=current_service.id, broadcast_message_id=broadcast_message.id),
   )}}
 
-  {{ live_search(target_selector='.multiple-choice', show=show_search_form, form=search_form, label='Search by name') }}
+  {{ live_search(target_selector='.govuk-checkboxes__item', show=show_search_form, form=search_form, label='Search by name') }}
 
   {% call form_wrapper() %}
-    {{ checkboxes(form.areas, hide_legend=True) }}
+    {{ form.areas }}
     {{ sticky_page_footer('Add to broadcast') }}
   {% endcall %}