Omitting the flag from optional-features enables the runtime deprecation notice.
Also introduces `ember-jquery-legacy` which can be used to migrate to the new behaviour early. Details at https://deprecations.emberjs.com/v3.x/#toc_jquery-event
Core does not appear to make use of `originalEvent` in Ember event handlers. When searching for `originalEvent` there are some matches which relate to our pan-events mixin, but this is our own implementation and not affected by this deprecation.
