Configure nginx to send pre-gzipped files to the client

Rails already creates little pre-gzipped `.gz` copies of all our assets
in the `public/assets` directory when we build. This configures nginx to
send those when available!

We weren't doing *any* gzip stuff before, so this helps a lot with those
bigger JS files, like the `wardrobe-2020` stuff. It's now at ~.5MB with
compression, which is still a bit big, but nowhere near as offensive as
the 4.5MB pre-anything, or 1.5MB post-minification, lol.

deploy/files/sites-available/impress.conf

@@ -17,6 +17,18 @@ server {
 
   root /srv/impress/current/public;
 
+  # Serve assets using their precompressed *.gz versions.
+  # The filenames contain content hashes, so they should be safe to
+  # cache forever.
+  # https://stackoverflow.com/a/6952804/107415
+  location ~ ^/assets/ {
+    gzip_static on;
+    expires     max;
+    add_header  Cache-Control public;
+    add_header  Last-Modified "";
+    add_header  ETag "";
+  }
+
   # Try serving static files first. If not found, fall back to the app.
   try_files $uri/index.html $uri @app;