Improve error handling

fix(strategy): #8 `UserWithIdStrategy` not working with int IDs

added unit tests for UserWithIdStrategy

Add Symfony 4.4 compatibility

See #5

Switched from Symfony components to Symfony contracts for dependencies
in composer.json

fix(config): use `%unleash.environment%` for `UNLEASH-APPNAME` in HttpClient configurations

instead of `%kernel.environment%` in
`DependencyInjection/UnleashExtension.php`

removed validation and normalization of `api_url` configuration node

Check global feature status