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.toml

@@ -80,6 +80,7 @@ uuid = { version = "0.8.2", features = ["v4"] }
 chrono = { version = "0.4.19", features = ["serde"] }
 chrono-tz = "0.6.1"
 time = "0.2.27"
+httpdate = "1.0"
 
 # Job scheduler
 job_scheduler = "1.2.1"