update from jquery.recurrenceinput.js
Configure ributtonExtraClass with 'allowMultiSubmit'. This prevent Plone's
  warnings against clicking the same submit button multiple times. There is no
  need to customize the FORMTMPL anymore when updating the javascript from
  jquery.recurrenceinput.js.
Let occurrences preview show the correct daynames and not by one day off.
  Fixes https://github.com/plone/plone.app.event/issues/69