Autofocus on input fields after routing

closes #3538, closes #3541
- The “autofocus” property isn’t considered by browsers if the element
is created after the page has been loaded. The Ember cookbook
recommends extending the input type with a manually triggered
autofocus, which I included here.
- This introduces the new helper “gh-focus-input”, an input field
gaining focus even when routing occurs.
Merge pull request #3581 from felixrieseberg/iss3541

Autofocus on input fields after routing