Hide notices on woo-dash pages

[justinshreve]

This PR hides standard WP admin notices (admin_notices) on woo-dash pages. This is only a partial solution. See #35, which still needs fully addressed once we have a design for how we actually want to present these.

This uses the solution Kevin came up with for synchrotron and WordPress/gutenberg#5590, however it also includes the wp-header-end div so all notices are caught (otherwise, some show up under the nearest h1/h2 🤦‍♂️ ).

To Test:

• Add a plugin that shows a notice. If you need one, you can use the following as a plugin:

<?php
/*
Plugin Name: Add Test Notice
Description: Adds a test notice.
Author: WooCommerce
Version: 1.0
*/

function test_add_notice() { ?>
	<div class="notice notice-warning is-dismissible">
		<p>
			<strong>Testing notice.</strong>
		</p>
	</div>
<?php }

add_action( 'admin_notices', 'test_add_notice' );

[timmyc]

This is testing out well for me. Thanks for getting this one in early Justin!