Add user controller and views for administrator
Remove unnecessary pages and routes for contestants
Add contest pages for administrator
Remove unnecessary contest pages for contestants
Add problems page for administrators
Remove unnecessary pages for contestants
Add spec of Admin::UsersController
Add spec of Admin::ContestsController
Refactor specs
Remove "Input download link" and show input/output in admin page
Remove "Back" link in /users/:id
Fix bug

Referred below bug
> A user created in the admin page has User id and Name, but User id seems to be shown as the name in /admin/users page, /users/:id page, and the status bar.
Remove debug code
Fix buf
Merge pull request #80 from murooka/71_admin_page

Split pages into admins' pages and contestants' pages