add a deprecation warning for using <template> inside Vaadin components

[vlukashov]

When using a <template> inside a Vaadin component like dialog, combobox, gird, etc after the support for Polymer <template> API is removed, I want to see a warning in the browser console.

<vaadin-combo-box .items="${tickers}">
 <template>
   <b>[[item.name]], <sub>[[item.number]]</sub>[[item.symbol]]</b><br>
   array index: [[index]]
 </template>
</vaadin-combo-box>

Case 1. if @vaadin/vaadin-template-renderer isn't imported

Browser console:

WARNING: <template> inside <vaadin-combo-box> is no longer supported. Import @vaadin/vaadin-template-renderer to enable compatibility (see https://vaad.in/template-renderer)

Case 2. if @vaadin/vaadin-template-renderer is imported, but the warning is not explicitly suppressed

See #313

The https://vaad.in/template-renderer short link should point to a page in vaadin.com/docs that describes the polymer template deprecation, how to use @vaadin/vaadin-template-renderer to enable compatibility, and how to suppress the deprecation warning.