Add font-awesome-rails
Add Submission, SubmissionsController

        submissions GET    /submissions(.:format)                  submissions#index
                    POST   /submissions(.:format)                  submissions#create
     new_submission GET    /submissions/new(.:format)              submissions#new
         submission GET    /submissions/:id(.:format)              submissions#show

submissions_problem GET    /problems/:id/submissions(.:format)     problems#submissions
Support tester_error
Add Submission#judge!

* Submission#judge_status: return :pending instead of nil
* Submission#judge_status: return always symbol
* Submission#judge!: judge itself using problem.test_examples and Judge
  class
Enable sidekiq

Sidekiq::Web is mounted on /sidekiq only logged in as staff.