[server] Only register "clean_sessions" looping tasks if repository has a tasks manager This prepares for calling Repository's _prepare_startup() in bootstrap() so that repository instances without a tasks manager running can still have their 'server_startup' hooks triggered (this is particularly useful for BFSS storages).

missing-dep-for-interpreter make => make | build-essential | dpkg-dev (usr/*/cubicweb/skeleton/debian/rules)
embedded-javascript-library usr/share/cubicweb/cubes/shared/data/jquery.js