Create a form extension for applying select2
Refactor select2 instantiation
Fix select2 allowclear when specifying the placeholder
Remove global select2 instantiation
Apply select2 to required fields
Merge branch 'master' into PIM-1620
Update unit test
Merge branch 'master' into PIM-1620
Update changelog
Apply select2 on the mass edit screen
Merge pull request #529 from akeneo/PIM-1620

Apply select2 only on required fields