Let users choose when to end a broadcast

Different emergencies will need broadcasts to last for a variable amount
of time. We give users some control over this by letting them stop a
broadcast early. But we should also let them set a maximum broadcast
time, for:
- when the duration of the danger is known
- when the broadcast has been live long enough to alert everyone who
  needs to know about it

This code re-uses the pattern for scheduling jobs, which has some
constraints that are probably OK for now:
- end time is limited to an hour
- longest duration is 3 whole days (eg if you start broadcasting Friday
  you have the choice of Saturday, Sunday and all of Monday, up to
  midnight)

app/templates/views/broadcast/preview-message.html

@@ -1,7 +1,8 @@
 {% from "components/button/macro.njk" import govukButton %}
 {% from "components/form.html" import form_wrapper %}
 {% from "components/page-header.html" import page_header %}
-{% from "components/page-footer.html" import sticky_page_footer %}
+{% from "components/page-footer.html" import page_footer %}
+{% from "components/radios.html" import radio_select %}
 
 {% extends "withnav_template.html" %}
 
@@ -28,7 +29,11 @@
   {{ broadcast_message.template|string }}
 
   {% call form_wrapper() %}
-    {{ sticky_page_footer('Submit for approval') }}
+    {{ radio_select(
+      form.finishes_at,
+      show_now_as_default=False
+    ) }}
+    {{ page_footer('Submit for approval') }}
   {% endcall %}
 
 {% endblock %}