Enable Studio Collab feature per project

The Studio Collab features need to be enable per project.

This feature is hardcoded at the moment.
See com.nuxeo.studio.client.model.ProjectSettings.isCollabEnabled():

https://github.com/nuxeo/nuxeo-studio/blob/master/nuxeo-studio-ui/src/main/java/com/nuxeo/studio/client/model/ProjectSettings.java#L206

This flag should be part of the project schema.

The flag is false by default : the feature is not enabled by default