gitweb: polish the content tags support

gitweb: polish the content tags support

This patch integrates the tag filtering CGI parameter into the framework
for parameter passing, dropping 'by_tag' and instead using query name 't'
and symbolic name 'ctag_filter'. Compatibility support for 'by_tag' query
name is retained.

This means that content tag links are now created using $cgi->a() and
the href() method, and that they now point to the proper action;
project_list in case of global content tags, forks in case of per-fork
content tags. Also any other arguments like sorting order of projects
are replayed within the links.

This patch also restores the ability to add tags if the ctags feature
is set to a POST action rather than a positive integer.

Signed-off-by: Petr Baudis <pasky@ucw.cz>
Signed-off-by: Kyle J. McKay <mackyle@gmail.com>