Merge pull request #655 from adericbourg/master

fix #651: handle deleted users.
Add missing jquery.placeholder.js asset in wro4j config
Merge pull request #656 from jxerome/master

Add missing jquery.placeholder.js asset in wro4j config
Correct incorrect == String equality.
Make verification of the right to modify user/group association more explicit and reduce code replication.
Merge pull request #657 from jxerome/equal-compare

Correct incorrect == String equality
Replace "null" management with optionals.