For small, personal self-hosted instances with no user signups, the fork
button is just a noise. This patch allows disabling them like stars can
be disabled too.
Disabling forks does not only remove the buttons from the web UI, it
also disables the routes that could be used to create forks.
Fixes#2441.
Signed-off-by: Gergely Nagy <forgejo@gergo.csillger.hu>
