[workflow] Utilities for declarative definition of workflows Closes #5337897

[server] Remove pseudo-handling of exceptions in Repository._delete_cascade_multi() It seems to me that we never want to pass on an exception here be it in "test" mode or not, so remove the last `except Exception:` clause. For instance, the actual implementation would pass on an IntegrityError and thus possibly hide it in a log message. Concerning the `except Unauthorized:`, as said by the log message this should not happen because we are within a `security_enabled(write=False)` context. So remove it as well. As far as I can tell, this strange handling of exceptions dates from 37668bf302f5 and there is no clear justification to it.

[web] fix os.rename usage on windows when destination exists os.rename on windows will raise OSError (or WindowsError subclass) when if destination file already exists. Also check that exception is EEXIST. There is an attempt to fix in f6ba947c but using IOError instead of OSError. Closes #14214794

[skeleton] Add "cubicweb.session.secret" to development.ini Commented as other "secret" settings. At least, now all settings are present in the development.ini file. [ci skip]

[pyramid] Include 'cubicweb.pyramid.predicates' where it is used It's only used in "rest_api" module.

[pyramid] Rename make_cubicweb_application function as config_from_cwconfig And update its docstring. This function does not actually "make a cubicweb application", it just builds a pyramid.config.Configurator instance from a CubicWeb config object.

[server] Reprase messages of db-check to avoid using "system" and "sources" And rather mention "entity type table" and "entities" table, which are clearer references.

[predicates] Fix reference to has_related_entities in partial_has_related_entities docstring

Fix possible double import of cubes modules When cubes using the new layout are imported with 'cubicweb_<cube>' and with 'cubes.<cube>', the same module is imported twice. Handle this by adding 'cubes.<cube>' to sys.modules when importing from 'cubicweb_<cube>'. Move load_module() to a sub class _CubesLoader to share informations computed in find_modules(). Don't handle subpackages in _CubesImporter and rely on normal import machinery instead. Add a test and use unittest from cubicweb.devtools.testlib which resolve to unittest2 on PY2 with assertLogs() method.

Make load_module() return existing module if present in sys.modules Otherwise the reload() builtin will not work correctly: https://www.python.org/dev/peps/pep-0302/#specification-part-1-the-importer-protocol