[web] Extract a base class out WebConfiguration This will be used to build a CubicWebPyramidConfiguration in following changeset.

[tox] Drop reference to removed init_instance.py file in flake8-ok-files.txt The file got removed in 1b9fd8bda1cd.

[web] Put 'use-uicache' option in level 2 So that users do not get prompted with this. Follow-up on c34590161082.

[server] Add a "scheduler" command to run repository scheduler This commands starts the repository scheduler as a standalone process that should complement a CubicWeb web instance running as a WSGI application. Added a log message in repository's shutdown method to help testing the command (i.e. make sure the method is called after the scheduler stopped). Related to #17057223.

[cwconfig] Do not instantiate a repo with a scheduler in repository method None of the caller of this method need the repository to have a scheduler and having a scheduler should be something that's explicitly requested.

[server] Make cubicweb/server/utils.py flake8-clean

[server] Drop utils's LoopTask and TasksManager classes not used anymore Related to 17057223.

[server] replace repository's tasks manager by a scheduler Repository() does not accept anymore a 'tasks_manager' argument but rather a 'scheduler' argument which is expected to be an instance of sched.scheduler class. The drop the _tasks_manager attribute of the repository and adjust all internal usages of it. In particular, in the 'repo_stats' service we do not export 'looping_tasks' statistics anymore as there's no way to retrieve this anymore from a web instance. Closes #17057223.