gitweb: Move static files into seperate subdirectory

Create a new subdirectory called 'static' in gitweb/, and move
all static files required by gitweb.cgi when running, which means
styles, images and Javascript code. This should make gitweb more
readable and easier to maintain.

Update t/gitweb-lib.sh to reflect this change.The install-gitweb
now also include moving of static files into 'static' subdirectory
in target directory: update Makefile, gitweb's INSTALL, README and
Makefile accordingly.

Signed-off-by: Pavan Kumar Sunkara <redacted>
Mentored-by: Christian Couder <redacted>
Mentored-by: Petr Baudis <redacted>
Acked-by: Jakub Narebski <redacted>
Acked-by: Petr Baudis <redacted>
Signed-off-by: Junio C Hamano <redacted>