Change the cgit layout

This modifies and hopefully improves the layout of all cgit pages:

  * Remove the header from all pages and replace it with a sidebar;
    most pages have sufficient width but many needs more height.

  * Add a dropdown-box to switch between branches, using a one-liner
    javascript to reload the current page in context of the selected branch.

  * Include refs found below refs/archives in the sidebar, appearing as a
    set of menuitems below a 'download' heading.

  * Include the brand new cgit logo

Signed-off-by: Lars Hjemli <hjemli@gmail.com>