web: add options to ignore css compilation and uicache

[skeleton] call includeme from new cube

[etwist] Do not call repository's start_looping_tasks anymore and warn about this We are about to drop this method from Repository class and replace it by a blocking alternative. This is not compatible with how things currently work in a Twisted server implementation. So do not start repository "looping tasks" in Twisted server anymore and issue a warning about this. If someone is interested in restoring the "all-in-one" behavior where the repository runs within a Twisted server, they may start by implementing repository looping tasks using a Twisted mechanism such as, e.g., http://twistedmatrix.com/documents/current/core/howto/time.html and eventually provide the repository with a compatible scheduler instance so that is can register its periodic tasks. At the moment, we lack resources to do this (and maintain the Twisted server of CubicWeb in general). Related to #17057223.

[test] Flake8-clean and use stdlib unittest in cubicweb/server/test/unittest_utils.py

[web] Don't raise ProcessFormError if there is no value specified a03376213747 introduced a slight modification in None / empty string values and with it a regression where an exception is raised on empty values, while we should simply consider no value is specified and return None.

[test] Cleanup imports in unittest_form Remove unused and reorder, + use bare unittest along the way.

Fix log level parameter not taken into account in cwsource log table An error in the jquery selector of the html widget controlling the log level was the cause of this bug. Closes #15772634 (grafted from c9129aae884b476455c8ed1ad802efc3a26503a5)