merge 3.18.4 into default
server/test/unittest_migractions fails due to interaction between new
constraint test (from 3.18) and composite deletion change (in 3.19).
usr/lib/PY_VERSION/*-packages/cubicweb/server/
usr/lib/PY_VERSION/*-packages/cubicweb/hooks/
usr/lib/PY_VERSION/*-packages/cubicweb/sobjects/
usr/lib/PY_VERSION/*-packages/cubicweb/schemas/
usr/share/cubicweb/migration/