index.html.in: support random and user-specified security codes

index.html.in: support random and user-specified security codes

The security code will now be set at page-load to a random two digit
string with characters taken from the set [0-9A-Z]. I've never
actually seen two consecutive digits (zero through nine) in the
official app; but it's good enough for now.

What's better is that you can pass ?code=YO via URL paramaters to set
the code to "YO". Since the code only changes once every day, a
one-way ticket revealing the code now essentially gets you a day pass.

Note: The resulting codes are not usually centered properly (yet).