server (tasks): refactoring: - moved `TasksManager` to `server.tasks.manager` - tasks modules now must have a `Task` class which will be instanciated by TasksManager - `server.tasks.task` has a basis for `Task` class - `Task.prepare` can now be asynchronous - `importlib` is now used to import tasks, instead of exec - tasks are now parsed/run after pages are imported - `server.BackendReady` is now a coroutine - type hinting for Task attributes