Fix more css/js issues

* moves fxos_feedback files out of the mobile/ directories
* nixes the mobile thanks which wasn't being updated and was out of date
* copies vendor_prefix.less to feedback/static/css/lib so we can use it
  there since jingo-minify runs in two different modes: like jekyll and
  hyde
* copies base/static/css/mobile/base.less to
  feedback/static/css/feedback_base.less since they're really two
  different things and separating them makes it way easier to restyle
  feedback forms without touching random parts of the site much like
  it's nice to get a footrub without also getting a lobotomy

r?