Admin dashboard

* Creat separate admin/questions controller
* Delete static admin page
* Change redirect to admin/dashboard when user is admin
* Add migration to rename admin_questions table into questions
We also need to rename:

app/models/admin_question.rb, to app/models/question.rb and change the first line from `class AdminQuestions < ActiveRecord::Base` to `class Questions < ActiveRecord::Base`, otherwise we don't have a model that can connect to the questions table.