Add lesson content: section on enums

* use cases
* benefits
  * readability
  * maintainability
  * speed
  * disk space
* how to implement
  * model
  * migration
    * warn about changing persisted data after
      change to enum declaration
* comparison to scopes
* class/instance methods exposed

implement changes suggested during review

* drop array model declaration
* rearrange lesson content
  * move up migrations
  * move down equivalent scopes
* use more pertinent example
  * light_switch => article (draft, published)
* unwrap lines
  * my editor defaulted to 80 char wrapping.
  * unwrapped all lines in enums/modified sections
* drop mention of/link to discrete values
* clarify enum definition
  * typically, not always, an int