Make {{partial}} and {{template}} helpers lookup templates using templateForName

This commit changes the implementation of the `{{partial}}` and `{{template}}` helpers to lookup templates using the view's `templateForName` method, like other template lookups, rather than directly from `Ember.TEMPLATES`. This will make it play nicely with container-registered templates. Fixes #2295
Merge pull request #2325 from lukemelia/use-container-for-partials

Make {{partial}} and {{template}} helpers lookup templates using templateForName