Twenty Seventeen: Make <th> styling in table block consistent between back end and front end.

Props ianbelanger.
Fixes #46568.

git-svn-id: https://develop.svn.wordpress.org/trunk@45051 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-content/themes/twentyseventeen/assets/css/editor-blocks.css

@@ -674,6 +674,7 @@ table.wp-block-table tr {
 	border-bottom: 1px solid #eee;
 }
 
+table.wp-block-table th,
 table.wp-block-table td {
 	border: 0;
 	padding: 0.4em;