Prevent eval time deprecations from vendor.

Since we are putting our deprecation polyfill and workflow handlers into the `my-app.js`, any deprecations triggered while evaluating `vendor.js` are still logged to the console.

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- [ ] Figure out how to move the ember-debug-handlers-polyfill to just after Ember loads
- [ ] Add `config/deprecation-workflow.js` to the beginning of `vendor.js` (via `contentFor('vendor-prefix')` instead of `contentFor('app-prefix')`).