move tabindex setting to modal controls

_includes/layouts/post.njk

@@ -36,7 +36,7 @@ layout: base.njk
 			<button class="close-dialog" autofocus aria-label="close the image modal">&times;</button>
 	    <img class="modal-img" src="/img/{{ image.src }}" alt="{{ image.alt }}">
 		</dialog>
-    <img tabindex="0" class="hero" src="/img/{{ image.src }}" alt="{{ image.alt }}">
+    <img class="hero" src="/img/{{ image.src }}" alt="{{ image.alt }}">
     {% endif %}
 
     {{ content | safe }}