backport default into stable: stable is now cw 3.8

backport stable into oldstable: oldstable is now cw 3.7

proper licensing information (LGPL-2.1). Hope I get it right this time.

[ms] fix planning bug with rewritten constants: when removing some source for a const due to relation constraints, we should propagate this change to other constants coming from the same original variable

[repo security] deal with rewriten constant nodes in check_read_access, necessary when repo is used as an external source

[doc] reorganize for chapter structure

[web auth] remove assertion, we may want to remove authentication info retreivers to disallow login from the web ui

merge

ask pyro group config questions for all-in-one config site when pyro is enabled in serverctl.py, the pyro questions are not asked for all-in-one configs, because this is webctl.py's responsibility. However for a all-in-one instance, repo_method is "in-memory" so the pyro questions would not be asked even if pyro had been explicitely enabled before.

enforce use of environment variables when running as a service CW_INSTANCES_DIR, CW_INSTANCES_DATA_DIR, CW_RUNTIME_DIR must be positionned at the system level. This will ensure that cubicweb-ctl commands issued on the computer will use the proper directories.