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)

tests/javascripts/radioSelect.test.js

@@ -99,7 +99,7 @@ describe('RadioSelect', () => {
         <legend class="form-label">
           When should Notify send these messages?
         </legend>
-        <div class="radio-select" data-module="radio-select" data-categories="${CATEGORIES.join(',')}">
+        <div class="radio-select" data-module="radio-select" data-categories="${CATEGORIES.join(',')}" data-show-now-as-default="True">
           <div class="radio-select__column">
             <div class="multiple-choice">
               <input checked="" id="scheduled_for-0" name="scheduled_for" type="radio" value="">