Output CloudFront-friendly headers for diffs (#1098)

This is part of web-monitoring#168 — the goal here is to make our diff cache less important so we can shrink it or even remove it (in favor of just having CloudFront do the job).

CloudFront needs a `Date` header to go with the `Last-Modified` and `ETag` headers from the `stale?` method, and works better still if we give it an actual cache age. This adds the `expires_in` helper which sets all the headers. I've also taken the strategy of setting a *relatively* short cache time but a much longer revalidation window when a stale response can be used, to balance concerns about updating the diff algorithm and effective long-term caching.

app/controllers/api/v0/diff_controller.rb

@@ -2,6 +2,12 @@ class Api::V0::DiffController < Api::V0::ApiController
   def show
     ensure_diffable
 
+    # Some front-end caches, like CloudFront, need the headers from *both*
+    # expires_in and stale? to cache most effectively.
+    unless Rails.env.development?
+      cache_time = params[:diff_version] ? 100.years : 1.day
+      expires_in(cache_time, public: true, stale_while_revalidate: 7.days, stale_if_error: 7.days)
+    end
     if stale?(etag: diff_etag, last_modified: Differ.cache_date, public: true)
       render json: {
         links: {