* upgrade Ember CLI to 3.13 and Ember Source + Ember Data to 3.12
Ember Source upgrade to 3.13 is blocked by Ember CP Validations,
which does not support Ember Source 3.13 yet.
Ember Data upgrade to 3.13 is blocked by Ember Data Model Fragments,
which does not support Ember Data 3.13 yet.
Also removes autofocus mixin and inlines the code as mixins are
considered an antipattern in latest Ember versions.
Removes some left overs in `"resolutions"` key. But that shouldn't have a real impact.
* 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.
