Handle old-style /goverment/uploads routes

This is currently handled by Whitehall but we're trying to move away
from Whitehall being a frontend application. The next best place for
this bit of code seems to be Government Frontend.

https://github.com/alphagov/whitehall/blob/master/test/functional/asset_manager_redirect_controller_test.rb

The code itself is used to handle assets which are still being requested
on their own URL before we migrated all assets over to Asset Manager.

app/controllers/asset_manager_redirect_controller.rb

@@ -0,0 +1,10 @@
+class AssetManagerRedirectController < ApplicationController
+  def show
+    asset_url = Plek.new.public_asset_host
+    if request.host.start_with?("draft-")
+      asset_url = Plek.new.external_url_for("draft-assets")
+    end
+
+    redirect_to host: URI.parse(asset_url).host, status: :moved_permanently
+  end
+end

config/routes.rb

@@ -11,6 +11,8 @@
 
   get "healthcheck", to: proc { [200, {}, [""]] }
 
+  get "/government/uploads/*path" => "asset_manager_redirect#show", format: true
+
   get "*path/:variant" => "content_items#show",
       constraints: {
         variant: /print/,

test/controllers/asset_manager_redirect_controller_test.rb

@@ -0,0 +1,22 @@
+require "test_helper"
+
+class AssetManagerRedirectControllerTest < ActionController::TestCase
+  setup do
+    Plek.any_instance.stubs(:public_asset_host).returns("http://asset-host.com")
+    Plek.any_instance.stubs(:external_url_for).returns("http://draft-asset-host.com")
+  end
+
+  test "redirects asset requests made via public host to the public asset host" do
+    request.host = "some-host.com"
+    get :show, params: { path: "asset", format: "txt" }
+
+    assert_redirected_to "http://asset-host.com/government/uploads/asset.txt"
+  end
+
+  test "redirects asset requests made via draft host to the draft asset host" do
+    request.host = "draft-some-host.com"
+    get :show, params: { path: "asset", format: "txt" }
+
+    assert_redirected_to "http://draft-asset-host.com/government/uploads/asset.txt"
+  end
+end