ToDo left:
refactor `datetimes` property of `create-options-datetime` component to
not cause whole list to be updated. This is not only a performance issue
but also UX: e.g. validation state is lost when another item is added.
part of #76
replaces ember-easy-form-extensions and ember-validations by
ember-cp-validations and ember-form-master-2000
dropped webshim for input type time; should be readded to improve
user experience in browser which does not support this input field
yes (e.g. Internet Explorer, Edge, Firefox)
