Make Assetic integration optional

[emodric]

In cases when Assetic is not present (e.g. eZ Platform 2.5) and after updating to PHP 7.3.7 you will get an exception:

PHP Fatal error:  During class fetch: Uncaught ReflectionException: Class Assetic\Factory\Loader\FormulaLoaderInterface not found in vendor/ezsystems/legacy-bridge/mvc/Templating/LegacyFormulaLoader.php:18
Stack trace:
#0 vendor/symfony/symfony/src/Symfony/Component/Debug/DebugClassLoader.php(156): require('vendor/asset...')
#1 [internal function]: Symfony\Component\Debug\DebugClassLoader->loadClass('eZ\\Publish\\Core...')
#2 [internal function]: spl_autoload_call('eZ\\Publish\\Core...')
#3 vendor/symfony/symfony/src/Symfony/Component/Config/Resource/ClassExistenceResource.php(78): class_exists('eZ\\Publish\\Core...')
#4 vendor/symfony/symfony/src/Symfony/Component/DependencyInjection/ContainerBuilder.php(371): Symfony\Component\Config\Resource\ClassExistenceResource->isFresh(0)
#5 vendor/symfony/symfony/src/Symfony/Component/DependencyInject in vendor/ezsystems/legacy-bridge/mvc/Templating/LegacyFormulaLoader.php on line 18

This is due to fixing https://bugs.php.net/bug.php?id=76980

This PR fixes it by making the Assetic integration optional, based on existence of Assetic\Factory\Loader\FormulaLoaderInterface interface.

[emodric]

Ping @kmadejski for review, this is kinda urgent for us.

[emodric]

Thank you @kmadejski !