[UX]/[DX] Add a fancy "copy to clipboard" button for views templates

A follow-up from https://github.com/backdrop/backdrop-issues/issues/1265

From @docwilmot

This is a great idea. How about we add one of those js "click this link/button to copy to clipboard" thingies as well?

From @quicksketch

Unfortunately the only way to do this (afaik) is to use a Flash-based widget. Normal JavaScript doesn't have access to write to the clipboard.

From @Dinornis

no Flash stuff, please.

From @jenlampton

How does GitHub do it?

<button aria-label="Copy to clipboard" class="js-zeroclipboard btn btn-sm zeroclipboard-button tooltipped tooltipped-s" data-copied-hint="Copied!" type="button">
  <svg aria-hidden="true" class="octicon octicon-clippy" height="16" version="1.1" viewBox="0 0 14 16" width="14">
    <path fill-rule="evenodd" d="M2 13h4v1H2v-1zm5-6H2v1h5V7zm2 3V8l-3 3 3 3v-2h5v-2H9zM4.5 9H2v1h2.5V9zM2 12h2.5v-1H2v1zm9 1h1v2c-.02.28-.11.52-.3.7-.19.18-.42.28-.7.3H1c-.55 0-1-.45-1-1V4c0-.55.45-1 1-1h3c0-1.11.89-2 2-2 1.11 0 2 .89 2 2h3c.55 0 1 .45 1 1v5h-1V6H1v9h10v-2zM2 5h8c0-.55-.45-1-1-1H8c-.55 0-1-.45-1-1s-.45-1-1-1-1 .45-1 1-.45 1-1 1H3c-.55 0-1 .45-1 1z"></path>
  </svg>
</button>