[backend] creates a base class for questions: free and choice.

The base class servers to create free questions and choice questions.
[frontend] with new question base class, now able to store participant results.
[backend] quiz.models: answer_must_match is mandatory field.
[backend] quiz.models: the content of an answer can be blank.

The case when the question is not a free input.
[frontend] main.services: config and ngResource are not used here.
Merge branch 'quiz_models_table_inheritance' into quiz_controller

* quiz_models_table_inheritance:
  [frontend] main.services: config and ngResource are not used here.
  [backend] quiz.models: the content of an answer can be blank.
  [backend] quiz.models: answer_must_match is mandatory field.
  [frontend] with new question base class, now able to store participant results.
  [backend] creates a base class for questions: free and choice.