One click revert for managers

Ignore-this: e3c9b5f941b2f1aa83ca375861203a2f
This patch adds another button for users with the $conf['manager'] role when
viewing an old revision. It allows them to revert to this revision with a
single click.

darcs-hash:20090911081833-7ad00-5a64feb7e3e1b37178295c290a6c97c3923e82e3.gz