CM6: Further fixes/improvements after testing

- Updated event naming to be "cm6" when codemirror-specific.
- Removed cm block border in md editor to prevent double bordering.
- Updated copy handling to fallback to execCommand.

resources/sass/_forms.scss

@@ -86,6 +86,8 @@
 .markdown-editor-wrap .cm-editor {
   flex: 1;
   max-width: 100%;
+  border: 0;
+  margin: 0;
 }
 
 .markdown-panel-divider {