Site Cloner: Warn before cloning into closed camp.

This helps to avoid unintentionally overwriting an old site with a new one when the organizer actually intends to clone _from_ the old site _to_ a new one.

public_html/wp-content/plugins/wordcamp-site-cloner/templates/site-control.php

@@ -7,6 +7,8 @@
 namespace WordCamp\Site_Cloner;
 defined( 'WPINC' ) or die();
 
+$wordcamp = get_wordcamp_post();
+
 ?>
 
 <div id="wcsc-cloner">
@@ -15,6 +17,18 @@
 		<span id="wcsc-sites-count" class="title-count wcsc-sites-count"></span>
 	</h3>
 
+	<?php if ( 'wcpt-closed' === $wordcamp->post_status ) : ?>
+		<div class="notice notice-warning">
+			<p>
+				<?php echo esc_html( sprintf(
+					// translators: %s is the name of the WordCamp.
+					__( '%s has already completed, are you sure you want to overwrite it with styles and settings from another site?', 'wordcamporg' ),
+					get_wordcamp_name()
+				) ); ?>
+			</p>
+		</div>
+	<?php endif; ?>
+
 	<div class="filters"></div>
 
 	<div class="wcsc-search">