users: allow anonymous users to use /profile.json.

Allow logged out users to call https://danbooru.donmai.us/profile.json.
This allows getting information on default settings and limits for
anonymous users.

May be a breaking API change if users were using the HTTP response code
from /profile.json to check if they were successfully logged in.

test/functional/users_controller_test.rb

@@ -228,9 +228,9 @@ class UsersControllerTest < ActionDispatch::IntegrationTest
         assert_redirected_to login_path(url: "/profile")
       end
 
-      should "return 404 for anonymous api calls" do
+      should "return success for anonymous api calls" do
         get profile_path(format: :json)
-        assert_response 404
+        assert_response :success
       end
     end