Normalize approach for remote service calls - ajax vs. fetch

[jabrah]

The fetch API doesn't have the same legacy issues that AJAX has, such as the madness with URL encoding data. However, Internet Explorer has zero support for this API. We could work around this, if necessary, by using a polyfill (https://github.com/ember-cli/ember-fetch).

Would require update to relevant services and tests.

Current usage of AJAX

• current-user.js
• autocomplete.js
• metadata-schema.js
• dashboard.js

Current usage of Fetch

• doi.js
• nlmta.js

[markpatton]

Should do the same for ember-fedora-adapter.