ember 2.18.2

[GavinJoyce]

Part of #462

This PR upgrades ember-model to use Ember 2.18.2. In order to minimise changes, I've tried to replicate the previous container logic with getOwner and setOwner.

TODO:

• 5 last five remaining tests (might be related to [BUGFIX lts] Refactor / fix error handling scenarios. emberjs/ember.js#15871)
• rebase and confirm 🍏
• confirm that this works for our own app in dev
• confirm that this works for our own app in production
• rename any remaining container variables to owner

[GavinJoyce]

I cut a custom build based on our Intercom fork and it seems to work well in dev: