api: support the expires_in param on all GET requests.

`expires_in` is an alternative to `expiry` that supports durations other
than days:

* https://danbooru.donmai.us/posts.json?expires_in=30s
* https://danbooru.donmai.us/posts.json?expires_in=5min
* https://danbooru.donmai.us/posts.json?expires_in=1hour
* https://danbooru.donmai.us/posts.json?expires_in=1week
* https://danbooru.donmai.us/posts.json?expires_in=1month

test/functional/application_controller_test.rb

@@ -199,6 +199,13 @@ class ApplicationControllerTest < ActionDispatch::IntegrationTest
       assert_equal("max-age=#{1.day}, private", response.headers["Cache-Control"])
     end
 
+    should "support the expires_in parameter" do
+      get posts_path, as: :json, params: { expires_in: "5min" }
+
+      assert_response :success
+      assert_equal("max-age=#{5.minutes}, private", response.headers["Cache-Control"])
+    end
+
     should "return the correct root element name for empty xml responses" do
       get tags_path, as: :xml