[repo] Fix register_user Make sure we have a connection to the db around user creation.

[test] use assertNotIn where appropriate Automatically generated with: find -name unittest_*.py -o -name test_*.py | xargs sed -i -e 's/assertFalse(\([^,]*\) in \([^,]*\))/assertNotIn(\1, \2)/' plus revert of a couple false matches.

[test] use assertIn where appropriate Automatically generated with: find -name unittest_*.py -o -name test_*.py | xargs sed -i -e 's/assertTrue(\([^,]*\) in \([^,]*\))/assertIn(\1, \2)/'

[doc] Add removal of old multi-sources to 3.19 release notes

[pkg] Bump to 3.19.0

[web/debug] remove code broken by session refactoring session.cnx and cnx.check() no longer exist.

[web/debug] Fix check for open sessions itervalues returns an iterator, which never evaluates to False. Regression from 0bb18407c053 "[toward py3k] rewrite dict.keys() and dict.values() (part of #2711624)".

[cwconfig] Remove useless 'ui-cube' configuration option (closes #3728904) The mechanism introduced could never actually be used as it brings dependency loops to the cube graph. Just get rid of it. This reverts commit 4032499c701e.

[test] Use RepoAccess.shell() in unittest_predicates

[devtools] add 'shell' method to RepoAccess, deprecate CubicWebTC's Lets us control the connection's lifetime.