cubicweb-container #2909372 migration from pre 1.0: refactor & provide helpers to make it possible [resolved]

in some cubes, one will migrate to container AND add new containerised entities

for this to work we provide:

* an helper function to use to make the target cube migrations believe container_parent (now gone) actually exist at least at fsschema level

* a shell script to cleanup afterwards the spurious container_parent relations

prioritynormal
typebug
done in1.0.3
load0.200
load left0.000
closed by#adb6026a737f [utils,migration] provide helpers to help migrating from pre 1.0 (closes #2909372)
patch[utils,migration] provide helpers to help migrating from pre 1.0 (closes #2909372) [applied]