Allow passing computed ES6 modules to #view.

Right now, it's possible to pass a computed property to the {{view}}
helper, but that only works if the property returns an `Ember.View`.

This change allows the computed property to return a string, which
the helper will now try to interpret as an ES6 module and resolve.
Merge pull request #4975 from drwrf/refactor/view-resolution

Allow passing computed ES6 modules to {{view}}.