Fixed models & schema annotations

Changes following migrations

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

Update asset structure with webpacker gem

Replace YARD documentation with Tomdoc ...

These changes use the YARD gem, but in the lighter Tomdoc format.

You can read more about Tomdoc here: https://tomdoc.org

Add specs for completing a Plan

Add extra validations for all models ...

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

Adding fixtures