Validation rules

- implemented structure for storing different types of validation rules
- actual implementation include presence, numericality, format and 1-column inclusion validations
- the data structure is described here:
https://github.com/unepwcmc/SAPI/wiki/Trade-Validation-Rules
- validation rules creation statements are in seeds.rb
- existing validation rules are listed at http://localhost:3000/trade/#/validation_rules