This updates to Ember 3.16 but leaves Ember Data at 3.12 so we don’t need
to use the model fragments beta. It can be reviewed on a commit-by-commit
basis: blueprint updates, fixes for test failures, and the removal of
now-deprecated partials.

It’s not a true update to Octane as that would involve turning on template-only
components by default, which breaks various things. We can accomplish that
separately and then add the edition setting to package.json.