[Intl] Refactored Locale component into two new components Icu and Intl

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | yes
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | #5279
| License       | MIT
| Doc PR        | symfony/symfony-docs#2312

The Intl component is now a simple drop-in replacement layer for the C intl extension. Install it via Composer and have it available automatically if the intl extension is not available.

Additionally, the component ships data from the ICU library which can be accessed through the methods:

```php
use Symfony\Component\Intl\Intl;

Intl::getCurrencyBundle()->...
Intl::getLanguageBundle()->...
Intl::getLocaleBundle()->...
Intl::getRegionBundle()->...
```

If the intl extension is installed, Composer will install the ICU data for the ICU version in the intl extension. If the intl extension is not installed, Composer will use stub ICU data for the latest ICU version (see `Intl::getStubIcuVersion()`).

See the [README](/bschussek/symfony/blob/intl/src/Symfony/Component/Intl/README.md) for more information.

Todo:

- [x] finish the Intl README file
- [x] update the Icu README file
- [x] update the documentation
- [x] make parameter `$locale` optional (default to `\Locale::getDefault()`) in resource bundle methods
- [x] remove `(Icu)?Version::compare` calls in the tests
- [x] solve deployment problem when trying to install incompatible symfony/icu version listed in composer.lock

Create the following branches in the [Icu component](https://github.com/symfony/Icu):

- [x] 1.0.x
- [x] 1.1.x
- [x] 1.2.x