serve: Rewrite of the Server-Sent Event code for build notifications. At the moment the server monitors the asset directories, and notifies the browser when an asset has changed and has been re-processed. * Fix issues around long-running requests/threads which mess up the ability to shutdown the server correctly with `CTRL-C` (see comments in code). * Move the notification queue to each SSE producer, to support having multiple pages open in a browser. * Add JS/CSS for showing quick notifications about re-processed assets. * Add support for hot-reloading CSS and pictures that have been re-processed.

<ul class="nav navbar-nav navbar-right">
    <li><a href="{{ pcurl('getting-started') }}">Getting Started</a></li>
    <li><a href="{{ pcurl('docs') }}">Documentation</a></li>
    <li><a href="{{ pcurl('api') }}">Code</a></li>
    <li><a href="{{ pcurl('support') }}">Support</a></li>
</ul>