Add back compatability shim for data-sizes at all times.

This comes after reverting 0163649 since we have to transform data-sizes attributes
no matter which version of WP is installed. Since this filter uses a simple `str_replace()`
instead of a preg_match, it's probably cleaner to add a separate filter than trying to
piggyback on our `wp_make_content_images_responsive()` filter, and the performance should be
similar either way.

wp-tevko-responsive-images.php

@@ -59,3 +59,20 @@ function tevkori_get_picturefill() {
 	wp_enqueue_script( 'picturefill', plugins_url( 'js/picturefill.min.js', __FILE__ ), array(), '3.0.1', true );
 }
 add_action( 'wp_enqueue_scripts', 'tevkori_get_picturefill' );
+
+/**
+ * Back compatability shim for 'data-sizes' attributes in content.
+ *
+ * Prior to version 2.5 a 'srcset' and 'data-sizes' attribute were added to the image
+ * while inserting the image in the content. We replace the 'data-sizes' attribute by
+ * a 'sizes' attribute.
+ *
+ * @since 3.0.0
+ *
+ * @param string $content The content to filter;
+ * @return string The filtered content with `data-sizes` repaced by `sizes` attributes.
+ */
+function tevkori_replace_data_sizes( $content ) {
+	return str_replace( ' data-sizes="', ' sizes="', $content );
+}
+add_filter( 'the_content', 'tevkori_replace_data_sizes' );