gitweb: JavaScript ability to adjust time based on timezone

gitweb: JavaScript ability to adjust time based on timezone

This patch is based on Kevin Cernekee's <redacted>
patch series entitled "gitweb: introduce localtime feature".  While
Kevin's patch changed the server side output so that the timezone
was output from gitweb itself, this has a number of drawbacks, in
particular with respect to gitweb-caching.

This patch takes the same basic goal, display the appropriate times in
a given common timezone, and implements it in JavaScript.  This
requires adding / using a new class, "datetime", to be able to find
elements to be adjusted from JavaScript.  Appropriate dates are
wrapped in a span with this class.

Timezone to be used can be retrieved from "gitweb_tz" cookie, though
currently there is no way to set / manipulate this cookie from gitweb;
this is left for later commit.

Valid timezones, currently, are: "utc", "local" (which means that
timezone is taken from browser), and "+/-ZZZZ" numeric timezone as in
RFC-2822.  Default timezone is "local" (currently not configurable,
left for later commit).

Fallback (should JavaScript not be enabled) is to treat dates as they
have been and display them, only, in UTC.

Pages affected:
* 'summary' view, "last change" field (commit time from latest change)
* 'log' view, author time
* 'commit' and 'commitdiff' views, author/committer time
* 'tag' view, tagger time

Based-on-code-from: Kevin Cernekee <redacted>
Signed-off-by: John 'Warthog9' Hawley <redacted>
Signed-off-by: Jakub Narebski <redacted>
Signed-off-by: Junio C Hamano <redacted>