Update deploy process to use progressive rollout

[dakkad]

Refactor dynamic includes of AMP validate to use a fixed version. Also change the roll-out process for the pinned version to support migrating traffic to new versions to test rolling forward.

The process should be repeated regularly to include ongoing changes to dependencies like the AMP validate dependencies rather than run-time updates which could result in unintended issues.

[ithinkihaveacat]

A progressive rollout seems like a pain to implement and document, I don't think we're gonna do anything like that in the short term! (Note that there is a staging server, although I do admit that does not move the project very far toward rollout and reliability best practices…)

I do want to fix the validator issue though; have created #97 for that.