[BUGFIX beta] Ensure {{link-to}} path observers are reregistered after render.

Prior to this change we called `registerObserver` on `init` so that `this.rerender` was triggered when the link text changes (see [here](https://github.com/emberjs/ember.js/blob/3a7f4ae7c0be07f064f31e1df3f06d38ff866f4c/packages/ember-routing/lib/helpers/link_to.js#L192-L196)).

This worked properly to trigger the first `rerender`, but since `registerObserver` tears down the observer on `willClearRender` (see [here](https://github.com/emberjs/ember.js/blob/3a7f4ae7c0be07f064f31e1df3f06d38ff866f4c/packages/ember-views/lib/views/view.js#L2273-2275)) and the observers were not set back up, we would only handle a single change.

This change ensures that the observers are setup after each time the view is rerendered.

**Please Note:** This issue exists in 1.2.0, but I was unsure if this is a large enough issue to cause a point release update.  Let me know if I should change the commit tag to `[BUGFIX stable]`.

Resolves https://github.com/emberjs/ember.js/issues/3925.