(bug 43981) disable live preview on user .js/.css pages

Disable live preview on user .js/.css pages, as there's no sane way of
"previewing" the scripts or styles without reloading the page.

Change-Id: I8750448c827eb965509158bb4a0cd8b0cf3c7a67
Merge "(bug 43981) disable live preview on user .js/.css pages"