Fix click on validate button when importing members in social workspace

Social collaboration tests fail randomly when importing members, and problem has been reproduced locally (but steps to reproduce are unknown for now).

There's an issue with the "validate" button activation: buttons are re-rendered, but this one is still disabled (some others are enabled) sometimes.

Attaching selenium tests showing the issue, as well as server logs.

This is visible here for instance: https://qa.nuxeo.org/jenkins/job/FT-nuxeo-master-selenium-social-collaboration-tomcat/1418/