Additionally set expires header when caching responses

Browsers are rather smart, but also dumb. This uses the `Expires` header
alongside `cache-control` to better prompt the browser to actually
cache.

Unfortunately, firefox still tries to "race" its own cache, in an
attempt to respond to requests faster, so still ends up making a bunch
of requests which could have been cached. Doesn't appear there's any way
around this.

Cargo.lock

@@ -3295,6 +3295,7 @@ dependencies = [
  "governor",
  "handlebars",
  "html5ever",
+ "httpdate",
  "idna 0.2.3",
  "job_scheduler",
  "jsonwebtoken",