Inserts a dummy DOM element that the widget uses to calculate the base URL for its assets.

Under standard setup, the RTM widget gathers its base URL by inspecting the src attribute of
the <script> used to load the library. Since we've got a custom setup that doesn't include
a <script> tag, we need to fake this by placing a DOM element with id="directlyRTMScript"
and a src with the appropriate base URL.

@see https://cloudup.com/cySVQ9R_O6S for the standard setup instructions we've modified
function insertDOM() {
	if ( null !== document.getElementById( 'directlyRTMScript' ) ) {
		return;
	}
	const d = document.createElement( 'div' );
	d.id = 'directlyRTMScript';
	d.src = DIRECTLY_ASSETS_BASE_URL;
	document.body.appendChild( d );
}