Add a fastboot config for field-guide > 3.0.1

[BlueCutOfficial]

PR #1225 introduced a bug in the demo app by upgrading the version of field-guide that was enforced.

History: When field-guide was updated to version 3.0.0 in #1273, the exact version 3.0.0 was enforced by removing ^ because the patch 3.0.1 was introducing an error related to the fetch definition that was not a priority to investigate at that time. #1225 introduced the bug by upgrading to 3.1.0, greater than 3.0.1.

This PR addresses the root cause of the issue, allowing us to keep field-guide 3.1.0 installed.

The meaning of the 3.0.1 patch is that fetch is now available in Node by default; it's part of the globals, so we no longer need a tool like ember-fetch to define the method. However, the way Fastboot works is peculiar, and the Node globals are not available by default when serving an app with Fastboot. Globals need to be passed explicitly to the Fastboot sandbox, which can be done through a configuration file fastboot.js.