css: Avoid displaying copy_codeblock button in previews.

Having this option in preview feels rather odd. The code here
would either be pasted from elsewhere (in which case it would
be in the clipboard already) or it could be copied from the
writebox just as easily.

static/styles/rendered_markdown.scss

@@ -469,6 +469,14 @@
     }
 }
 
+.preview_content .copy_codeblock {
+    /* We avoid displaying copy_codeblock button in previews, because it
+       feels odd given that you can just copy-paste the code out of
+       the "edit" state.  We may change this decision when we add
+       menu options for viewing the code in a coding playground. */
+    display: none;
+}
+
 @media (max-width: 600px) {
     .rendered_markdown .message_embed {
         height: auto;