Add Classifications and Resource validations

- Turn `has_and_belongs_to_many` into `has_many :through`: now,
resources and sections are related through Classifications.
- Refactor usage of jsonb column to use ActiveRecord validations
- Attention! store_accessor:

NOTE: If you are using structured database data types (eg. PostgreSQL hstore/json, or MySQL 5.7+ json) there is no need for the
serialization provided by .store. Simply use .store_accessor instead to generate the accessor methods. Be aware that these
columns use a string keyed hash and do not allow access using a symbol.

NOTE: The default validations with the exception of uniqueness will work. For example, if you want to check for uniqueness with
hstore you will need to use a custom validation to handle it.

https://api.rubyonrails.org/classes/ActiveRecord/Store.html

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