gitweb: Add beginnings of cache administration page (proof of concept)
Currently cache administration page ('cache' action) shows estimated
size of cache, and provides link to clearing cache.
Cache administration page is visible only on local computer; the same
is true with respect to ability to clear cache. Those are bare
beginnings of autorization framework.
If you can use cache administration page, you will see 'admin' link at
the botom of the page to it.
Signed-off-by: Jakub Narebski <jnareb@gmail.com>