Say ‘done’ not ‘back’

We use ‘back’ to label a control which navigates to a previous page now.
It could be confusing to keep this control labelled the same way. And
in the future the folder permissions interaction definitely shouldn’t be
using ‘back’, because it suggests undoing the selection you’ve made.

app/assets/javascripts/radioSelect.js

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