Merge pull request #1664 from akeneo/PIM-3279-stable

PIM-3279: Add an ajax filter
PIM-3266: add Behat scenarios to detect the problem
PIM-3266: do not flush skipped items during an import
Merge pull request #1663 from akeneo/PIM-3266

PIM-3266: do not flush skipped items during an import
PIM-3221: Fix bug on variant group import

*Fix bug on variant group import, possibility to update variant
groups's attributes on import and it shouldn't be possible (currently it display a
error message)

*Improved the behats