[Validator][Email] - Strict validation and soft dependency

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | yes
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #1581, #4930 
| License       | MIT
| Doc PR        | https://github.com/symfony/symfony-docs/pull/3469

TODO
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- [x] submit changes to the documentation
- [x] document the BC breaks
- [x] finish the code
- [x] gather feedback for my changes

In #1581 @bschussek suggested to pass the strict_email config to  `Validator\EmailValidator::__construct($strict)`, I did not put it there yet since the constraint can receive that configuration each time the constraint is used despite the fact of the global configuration. This could lead to some logic in the constructor and I wanted first to integrate the strict validator.

BC Break
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I'm not sure of this, but as a soft dependency is added and now some emails that where valid before no longer are I thought it is.