First Doctrine cache usage in our app, added a LocalesManager

* added a LocalesManager: we don't have any global app locales file, but each Plugin now comes with it own bag of locales
* added a EventsManager: Plugins with "$app->before()" or "$app->after()" hooks should use the new "services" section of their YAML file to declare and implement these hooks
* the PluginsFinder, PluginsManager & LocalesManager can store their computed data into the app cache
* each Plugin must now have an ID
* the "logger" Service is now a Bootstrap Service
* HTML page header links are now added by Plugins instead of being hardcoded in the Twig template
* added 2 optional "general/htmlHooksPriority"  & "general/actionsUrlsPrefix" Plugins config settings
* added a "util" core Plugin
* first "PhpBb data import" Plugin skeleton
* some additional refactoring & optimizations