Prevent double text selection in the editor

closes TryGhost/Ghost#9356
- it was possible to start a selection from outside of CodeMirror's control, this was confusing because text appeared to be selected but wasn't
- disable user selection on the outermost CodeMirror wrapper so that the selection behaves as it does with a normal textarea where the selection needs to start from inside the content area

app/styles/layouts/editor.css

@@ -450,6 +450,8 @@
     overflow: visible;
     padding: 0;
     background: transparent;
+    /* Prevent confusing select behaviour, see https://github.com/TryGhost/Ghost/issues/9356 */
+    user-select: none;
 }
 
 .gh-editor .CodeMirror-cursor {