Use jsdelivr.net as CDN server

Those guys [0] provide a lot of javascript libraries, so it is now
possible to use a single provider (jsdelivr.net) instead of multiple
ones (bootstrapcdn.com, google.com ecc...). This should at least reduce
the DNS query and make the code cleaner.

colorbox.css has been merged inside silverstrap.css and colorbox.jss is
now feeded from jsdelivr.

[0] http://www.jsdelivr.com/
Remove class from checkbox <input>

Bootstrap expects a plain <input type="checkbox"> for checkbox fields,
so removing bogus classes from the template. Leaving other attributes
intact tough (required, value, name etc...).