Fixed email-cta button text not being selectable in Firefox

refs https://github.com/TryGhost/Team/issues/928

- the `.gh-btn` styling has `user-select: none` which was preventing Firefox from allowing selection using the mouse

ghost/admin/app/styles/layouts/editor.css

@@ -981,4 +981,5 @@ figure {
     padding: 4px;
     border: 0;
     text-align: center;
+    user-select: auto;
 }