Use a larger alphabet for shortcodes

Allow a customisable alphabet instead of using base_convert(). By
default, use one with 64 characters. The characters were selected to be
easy to transcribe by hand, with "1", "l" and "I" omitted.

If there are any existing installations of UrlShortener, backwards
compatibility with existing URLs can be achieved with

$wgUrlShortenerIdSet = '0123456789abcdefghijklmnopqrstuvwxyz';

Change-Id: I58b53715d0822be1bf67633bb156b24ce53ba26c