gitweb: Support for tag clouds

gitweb: Support for tag clouds

The "Content tags" (nothing to do with usual Git tags!) are free-form
strings that are attached to random projects and displayed in the
well-known Web2.0-ish tag cloud above project list.

The feature will make use of HTML::TagCloud if available, but will
still display (less pretty) list of tags in case the module is not
installed.

The tagging itself is not done by gitweb - user-provided external
helper CGI needs to be provided; one example is the tagproj.cgi
of Girocco. This functionality might get integrated to gitweb
in the future.

The tags are stored one-per-file in ctags/ subdirectory. The reason
they are not stored in the project config file is that you usually
want to give anyone (even CGI scripts) permission to create new tags
and they are non-essential information, and thus you would make
the ctags/ subdirectory world-writable.

Signed-off-by: Petr Baudis <petr.baudis@novartis.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>