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.

app/controllers/users_controller.rb

@@ -57,7 +57,7 @@ class UsersController < ApplicationController
     elsif request.format.html?
       redirect_to login_path(url: profile_path)
     else
-      raise ActiveRecord::RecordNotFound
+      respond_with(@user, methods: @user.full_attributes, template: "users/show")
     end
   end