DEV: Switch over to a fork of ember-rails (PR #10603)

We are switching over to a fork because we are currently on a pinned version of ember-rails 0.18.5 which is pretty old. Upgrading to the latest version causes many things to break which isn’t really worth the time to debug while we plan to completely switch over to ember-cli somewhat soonish. Our fork contains a single cherry-pick commit

which will fix an issue when running the rails g migration command and it spits out a bunch of deprecation warnings.