301 redirection for Let's Encrypt acme challenges

This is actually part of gitlab-org/gitlab-ce#28996, but for visibility sake, I'm putting it in a separate issue here.

Proposal

Most of the Let's Encrypt stuff will be handled by gitlab-rails (see gitlab-org/gitlab-ce#28996). But gitlab-pages should redirect from the custom URL to the gitlab URL:

GET example.com/.well-known/acme-challenge/:acme_key

=>

301
Location: gitlab.com/-/acme-challenge/mydomain.com/:acme_key