PERF: Improve Accept header handling for stylesheets and theme-js (#19357)

The default behavior for Rails is to vary the response of an endpoint based on the `Accept:` header, and therefore it returns a `Vary:` header on responses. This instructs browsers and intermediate proxies to key their caches based on the value of the request's `Accept` header. In some cases (e.g. Akamai), the presence of a `Vary` header is enough to prevent caching entirely.

This commit restructures the Rails route definitions so that:
1. The "format" segment of the route is 'required'
2. The "format" segment of the route is constrained to a single value (e.g. `js` or `css`)

Now that the routes are guaranteed to have a `:format` segment, Rails will always prioritize that over the `Accept` header, and will therefore omit the `Vary` header.

Request specs are also added to test this behaviour for both stylesheets and theme-javascripts.

spec/requests/theme_javascripts_controller_spec.rb

@@ -75,6 +75,25 @@ RSpec.describe ThemeJavascriptsController do
         //# sourceMappingURL=#{digest}.map?__ws=test.localhost
       JS
     end
+
+    it "ignores Accept header and does not return Vary header" do
+      js_cache_url = "/theme-javascripts/#{javascript_cache.digest}.js"
+
+      get js_cache_url
+      expect(response.status).to eq(200)
+      expect(response.headers["Content-Type"]).to eq("text/javascript")
+      expect(response.headers["Vary"]).to eq(nil)
+
+      get js_cache_url, headers: { "Accept" => "text/html" }
+      expect(response.status).to eq(200)
+      expect(response.headers["Content-Type"]).to eq("text/javascript")
+      expect(response.headers["Vary"]).to eq(nil)
+
+      get js_cache_url, headers: { "Accept" => "invalidcontenttype" }
+      expect(response.status).to eq(200)
+      expect(response.headers["Content-Type"]).to eq("text/javascript")
+      expect(response.headers["Vary"]).to eq(nil)
+    end
   end
 
   describe "#show_map" do