[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.