gitweb.js: Add UI for selecting common timezone to display dates

gitweb.js: Add UI for selecting common timezone to display dates

This will modify HTML, add CSS rules and add DOM event handlers so
that clicking on any date (the common part, not the localtime part)
will display a drop down menu to choose the timezone to change to.

Currently menu displays only the following timezones:

  utc
  local
  -1200
  -1100
  ...
  +1100
  +1200
  +1300
  +1400

In timezone selection menu each timezone is +1hr to the previous.  The
code is capable of handling fractional timezones, but those have not
been added to the menu.

All changes are saved to a cookie, so page changes and closing /
reopening browser retains the last known timezone setting used.

[jn: Changed from innerHTML to DOM, moved to event delegation for
onclick to trigger menu, added close button and cookie refreshing]

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