* Adds a loading spinner to form buttons as long as submission is pending.
* Does some refactoring of form navigation buttons.
* Updates expected bundle size.
Drops ember-radio-buttons addon and uses a plain input element instead. ember-radio-buttons doesn't seem to be maintained anymore and required jQuery. Also plain input element seems to be much easier to maintain.
Replaces ember-ajax by ember-fetch as ember-ajax is build on top of jQuery.ajax().
This reduces the bundle size by 25 KB (JavaScript) after gzip. Updating the size limit accordingly.
