Add a basic sample application Related to #4291173

Integration pyramid and cubicweb authentication. We use pyramid sessions to store the cubicweb sessionid so we can reuse it when needed, or regenerate it if it was lost. The cubicweb sessionid is obtained from a login in the repo OR directly from the user identified by pyramid. Related to #4291173

[doc] Escape _ in README.md So that the Markdown is okay.

Initial implementation Set up a default route that passes requests to a cubicweb instance. The requests are wrapped in an adequate adapter so that cubicweb works with no change. Related to #4291173

Project Structure Related to #4291173

[tox] Have less test environments This is to reduce the load on CI server because of parallel clones of the repository in Docker containers. Only keep "server", and "web" environments separated. All other prior environments are in "misc". To avoid duplicate entries in requirements files, move them all in a "requirements" directory (this appears to be a "common" practice, see e.g. Celery). Adjustments in tests: * Rename cubicweb/hooks/test/unittest_notification.py so that it does not conflict with cubicweb/sobjects/test/unittest_notification.py during test discovery as they would have the same module name but different __file__ attribute. * Add "comment" cube to the list of expected cubes in unittest_cwconfig.py as this cube is pulled by requirements/test-mist.txt. Closes #15440662.

[repo] Don't crash on start when fs schema is missing some db schema entities This occurs usually while developping and we don't want systematically to rebuild the database to start the instance or run e.g. i18ninstance.

Let configuration option be overridden by environment variables Related to #13889793.

[tox] Add a dummy test-results.xml file in check-manifest and flake8 environments This is to work around Jenkins Junit plugin that does not apparently account for the absence of this file (though it accepts it to be empty).

[tox] Basic flake8 config