Quiz entity form

- [x] Define form
- [x] Validate
- [x] hook_field_extra_fields()
- [x] If admin selects "Use the administration theme when editing or creating content" at /admin/appearance, use admin theme for /quiz/* (admin pages).
- [x] If there's only one quiz type, /quiz/add is a form; If not, it should be a list of quiz type links.
- [ ] Submit
  - [x] Save quiz properties
  - [x] Check entity revision options
  - [ ] Remember as global
  - [ ] Remember my settings

#### Bugs

- [x] Quiz entity's 'Result feedback' options are not saved.