[workflow test] don't use session.user, subject to internal changes

[db creation test] more testing of db initialization: call build_db_cache and ensure admin user exists

[source synchronization hooks] Fix/enhance system source hooks They are broken if launched during repository initialization (this was not the case yet, but will be soon...). Add additional checks to ensure one doesn't try to store system source config in the database, as it will be ignored in favor of the "sources" file.

[validation error] refactor validation error handling so translation is done on the web side Users should now use cubicweb.validation_error helper function that will activate the feature with other handy behaviours. Also test testing for message in errors should call exception.tr(unicode) before comparing. Using bare ValidationError keep backward compat.

[repo cleanup] drop code moved to querier by 7e264ce34cd4

[repo cleanup] drop code moved to querier by 7e264ce34cd4

Added tag cubicweb-debian-version-3.15.4-1 for changeset 70cb36c826df

Added tag cubicweb-version-3.15.4 for changeset 63260486de89

[facets] add link 'focus on selection' (closes #525277)

[server/utils] catch ValueError from password verification passlib can raise ValueError when it can't recognized a hash. Treat that as a wrong password.