gitweb: refactor author name insertion

Collect all author display code in appropriate functions, making it
easier to extend these functions on the CGI side.

We also move some of the presentation code from hard-coded HTML to CSS,
for easier customization.

A side effect of the refactoring is that now localtime is always
displayed with the 'at night' warning.

Signed-off-by: Giuseppe Bilotta <redacted>
Signed-off-by: Junio C Hamano <redacted>