Twenty Sixteen: Add border-radius to avatar images in the editor.

This ensures that avatars design in the Post Author or Avatar blocks in the editor matches the front end.

Props pitamdey, sabernhardt, poena, shailu25.
Fixes #59253.

git-svn-id: https://develop.svn.wordpress.org/trunk@57577 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-content/themes/twentysixteen/css/editor-blocks.css

@@ -634,6 +634,13 @@ Description: Used to style blocks in the editor.
 	margin-bottom: 16px;
 }
 
+/* Avatar & Post Author */
+
+.editor-styles-wrapper .avatar,
+.wp-block-post-author__avatar img {
+	border-radius: 50%;
+}
+
 /*--------------------------------------------------------------
 7.0 Blocks - Colors
 --------------------------------------------------------------*/