gitweb: Refactor generating of long dates into format_timestamp_html

It is pure refactoring and doesn't change gitweb output, though this
could potentially affect 'summary', 'log', and 'commit'-like views
('commit', 'commitdiff', 'tag').

Remove print_local_time and format_local_time, as their use is now
replaced (indirectly) by using format_timestamp_html.

While at it improve whitespace formatting.

Inspired-by-code-by: Kevin Cernekee <redacted>
Signed-off-by: Jakub Narebski <redacted>
Signed-off-by: Junio C Hamano <redacted>