Fix installer and updater for use with backend JS

- include Modernizr in the installer as well (needed for no-js/js)
- reference backend JS before body instead of in head
- added container markup for installer error msgs
- some styling for tables in the installer
- remove obsolete no-js/js styles for JS-less installer
- compensate for the lack of iconfonts (due to the style.css not
  being piped through serendipity.css.php in the installer) by
  using different markup which works with system fonts