Add a back button to the scheduler

We’ve seen in research a user getting stuck playing with the
scheduler. They picked a day, but then didn’t want to choose one of the
options for that day. There’s no way to do this except pick a day and
then un-pick it.

What they ended up doing was clicking the grey back button, which took
them back to the previous page, making them upload their file again.

This commit adds a ‘back’ link for the scheduler. ‘Back’ seems like
sensible naming because that’s the thing that the user tried to click,
and the UI of a link matches the thing they clicked to get into this
situation.

app/assets/javascripts/radioSelect.js

@@ -30,6 +30,7 @@
             <label for="{{id}}">{{label}}</label>
           </div>
         {{/choices}}
+        <input type='button' class='js-reset-button js-reset-button-block' value='Back' />
       </div>
     `),
     'chosen': Hogan.compile(`