gitweb: Make JavaScript ability to adjust timezones configurable

gitweb: Make JavaScript ability to adjust timezones configurable

Configure JavaScript-based ability to select common timezone for git
dates via %feature mechanism, namely 'javascript-timezone' feature.

The following settings are configurable:
* default timezone (defaults to 'local' i.e. browser timezone);
  this also can function as a way to disable this ability,
  by setting it to false-ish value (undef or '')
* name of cookie to store user's choice of timezone
* class name to mark dates

NOTE: This is a bit of abuse of %feature system, which can store only
sequence of values, rather than dictionary (hash); usually but not
always only a single value is used.

Based-on-code-by: John 'Warthog9' Hawley <warthog9@eaglescrag.net>
Helped-by: Kevin Cernekee <cernekee@gmail.com>
Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>