[8.x] Added declined and declined_if validation rules

[Frozire]

Adds declined and declined_if validation rules.

Reverse off the accepted and accepted_if rules that require 'yes', 'on', '1', 1, true, 'true', these rules require a 'no', 'off', '0', 0, false, 'false' value.

As these functions are only new additions, they should not break anything.

Documentation PR: laravel/docs#7425
Laravel PR: laravel/laravel#5723

[taylorotwell]

Can you explain the use case from your application for this?

[Frozire]

Can you explain the use case from your application for this?

We are running a financial application where users answer yes, and no selectors, such as are you registered as a bad payer or do you have existing debt. We could solve our issue by reversing the question's phrasing, e.g. "not_registered_as_a_bad_payer", and using the accepted rule but this reduces intent.

At the same time, we allow financial instances to select their configurations of what they allow or disallow. This rule could help us align validation closer to the original intent and limit developer or interpretation errors when used to compare each input to each business's configuration; simply by avoiding a "semi-double negative".

[taylorotwell]

Thanks for contributing to Laravel! ❤️