…ts and Range (webmozart)

This PR was merged into the 2.6-dev branch.

Discussion
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[Validator] Added date support to comparison constraints and Range

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #3640, #7766, #9164, #9390, #8300
| License       | MIT
| Doc PR        | symfony/symfony-docs#4143

This commit adds frequently requested functionality to compare dates. Since the `DateTime` constructor is very flexible, you can do many fancy things now such as:

```php
/**
 * Only accept requests that start in at least an hour.
 * @Assert\GreaterThanOrEqual("+1 hours")
 */
private $date;

/**
 * Same as before.
 * @Assert\Range(min = "+1 hours")
 */
private $date;

/**
 * Only accept dates in the current year.
 * @Assert\Range(min = "first day of January", max = "first day of January next year")
 */
private $date;

/**
 * Timezones are supported.
 * @Assert\Range(min = "first day of January UTC", max = "first day of January next year UTC")
 */
private $date;
```

Commits
-------

60a5863 [Validator] Added date support to comparison constraints and Range