Implement EZP-20039: Automate install steps in composer post install/update cmd

This commit adds Core & Legacy Composer ScriptHandlers.

This is for use in https://github.com/ezsystems/ezpublish-community/pull/34
Allows legacy assets to be symlinked and prod assets to be dumped as part of
composer install / update command to reduce amounts of steps needed for install.

See issue for more info as well: https://jira.ez.no/browse/EZP-20039
Merge pull request #380 from ezsystems/composer_script_handlers

Add Core & Legacy Composer ScriptHandlers