Update template conversion to use Polymer.html template tag

[justinfagnani]

Assuming this proposal goes through: Polymer/polymer#4937

We'll want to output:

import {html, Element as PolymerElement} from '@polymer/polymer/polymer.js';

class MyElement extends PolymerElement {
  template = html`...`;
}

[kevinpschaaf]

@justinfagnani @FredKSchott Want to bump this up on the radar since Polymer/polymer#4971 is about ready to land. We'll likely make a 2.4-pre.1 pre-release and hold the feature until modulizer is updated with this change.