Add new privacy policy and terms of use

We are updating the privacy policy and terms of use as part of opening up
GitLab. Add these to the website and provide a page covering the need for
the change and providing links to the new and old documents.

Add a banner to the home page highlighting this change. We will need to
give 30 days notice before fully affecting the change as it's stated we
will do that in the existing privacy policy.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>