apply config/fastboot.js the same way that ember-cli-fastboot does

[mansona]

This lifts the implementation for discovering a config/fastboot.js file directly from ember-cli-fastboot: https://github.com/ember-fastboot/ember-cli-fastboot/blob/master/packages/ember-cli-fastboot/index.js#L386

Essentially since EmberData 4.12 you can't use prember because you need to provide a bunch of globals like AbortController and we aren't currently loading the fastboot.js config file that works with ember-cli-fastboot so it's impossible to run.

This will also fix #77 and #25 because you would just configure the sandbox globals the same way that ember-cli-fastboot tells you to 👍

Fixes #77
Fixes #25
Fixes #76

[mansona]

I did a bit of work to get CI green again #79 👍 I'll rebase this once that is merged