[BUGFIX] Deprecate manually assigned templates for Components.

In most circumstances a Component does not have a template or templateName property. In a component the block passed when calling it (in a handlebars template) is interpreted as its template, and a template that was registered as `components/my-component` becomes its layout. This was recently fixed in 1.2.0 (with the `container-renderables` feature), but many applications already existed that manually register their components (via `Ember.Handlebars.helper`).

This change allows usage of `template` and `templateName` properties on a component, but generates a deprecation warning if you do so.

Addresses the issue reported by @bradleypriest in [#3862](https://github.com/emberjs/ember.js/pull/3862#issuecomment-29963202).