Deploy an open sandbox server at gateway.metagov.org

[thelastjosh]

I'm surprised this isn't already an issue! For a while, we've been meaning to have an open sandbox server that allows people to go to gateway.metagov.org and try out PolicyKit + Gateway permissionlessly. So we need to deploy a new copy of PolicyKit. Farhaan originally worked on a Docker image to support that—however setting up a working PK instance requires more than just the Docker image. That process is currently a bit murky; only @amyxzhang really knows how to do it. So this issue is related to better documentation for PolicyKit.

It will also need more content for the login page—I think @CommitsVortex has already produced some of this writing.

I haven't assigned this to anyone since we need to add capacity and improve documentation before we tackle this.