github: enable lockdown of issues and merge requests

Libvirt uses GitHub as an automated read-only mirror. The goals were to
have a disaster recovery backup for libvirt.org, a way to make it easy
for people to clone their own private copy of libvirt Git, and finally
as a way to interact with apps like Travis.

The project description was set to a message telling people that we
don't respond to pull requests. This was quite a negative message to
potential contributors, and also did not give them any guidance about
the right way to submit to libvirt. Many also missed the description and
submitted issues or pull requests regardless.

It is possible to disable the issue tracker in GitHub, but there is no
way to disable merge requests. Disabling the issue tracker would also
leave the problem of users not being given any positive information
about where they should be reporting instead.

There is a fairly new 3rd party application built for GitHub that
provides a bot which auto-responds to both issues and merge requests,
closing and locking them, with a arbitrary comment:

   https://github.com/apps/repo-lockdown



This commit adds a suitable configuration file for libvirt, which
tries to give a positive response to user's issue/pullreq and guide
them to the desired contribution path on GitLab.

Reviewed-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>