Anonymous user browsing of some pages

The whole site's behind a user login page, but perhaps we could allow some pages to be public, eg gene and variant pages?

Would be nice to have before paper comes out, so people can use it a bit before registering - could be just a general annotation / reference site

Need to handle menus I think as the first step: see https://github.com/SACGF/variantgrid_private/issues/2911

Should do behind a setting so that can disable for Shariant etc