[#62454030] added some constraints to the pairing table, just because it gets on my nerves when a table has no constraints :)
keep trade related factories together
[Fixes #62454030] properly import blank pairings & consider them in validations
Merge branch 'develop' into optional_unit_with_some_trade_codes

Conflicts:
	app/models/trade/inclusion_validation_rule.rb
Merge pull request #156 from unepwcmc/optional_unit_with_some_trade_codes

Optional unit with some trade codes