Account for external 2021 path in the nginx redirect

[maeve-fpf]

Status

With the changes in #59 to handle //, it doesn't really make sense to try to handle trailing slash redirects over in ingress-nginx. Let's have the internal static-file nginx be responsible, and configure it to know its external URL.

This is all kind of a mess because we're translating /https-everywhere-2021 on the outside to /https-everywhere in here. I'm putting this right into the nginx.conf on the assumption that this URL is temporary and we will get rid of it pretty soon. But if we should add some sort of runtime configuration, I can look into that.

Review Checklist

N/A - Visual review and see next section

Post-Deployment Checklist

A curl of http://staging.securedrop.org/https-everywhere-2021 should return location: http://staging.securedrop.org/https-everywhere-2021/.

[echoesactiii]

Working as described for me!