Add html to LitElement.prototype

[cdce8p]

This PR maps the html function form lit-html to LitElement.prototype.html to be accessible within custom_ui components when overriding existing class implementation.

I've added the mapping inside src/layouts/app/home-assistant.js for now, but am happy to move it if there is a better place for it.

Example

const ExistingRow = customElements.get('existing-row');
// 'existing-row' being the name of the customElement

class MyFancyRow extends ExistingRow {
  render() {             // overrides existing render implementation
    return this.html`    // use `this.html`
      ...
    `;
  }
}

Fixes: #2107
CC: @balloob

[zsarnett]

@thomasloven

[thomasloven]

As much as I'd love to pull a fast one and just sneak-merge this, I would like to hear what @balloob has to say, since I know he doesn't really like putting stuff in the global scope.

Also, why only lit-html? Why not LitElement too?

Edit: Wait... this won't put anything in the global scope, will it?

[balloob]

What can go wrong 🙈

[cdce8p]

@balloob Thanks for the help!