Fix for ensuring that Questions that are option based must have atleast one non-blank text entry. ...

- Validation added for option based questions to ensure text field non-blank.
    - Validation message OPTION_PRESENCE_MESSAGE added to app/models/concerns/validation_messages.rb.
    - Added ':allow_nil => true' to
            delegate :option_based?, to: :question_format, :allow_nil => true
     in Question model to prevent NoMethodError if question_format is nil.

    Fix for #1725

Add extra validations for all models ...

- Includes specs using the Shoulda gem
- Includes a refactoring of update_numbers to a "sortable" abstraction