check that participants for parallel review are added.

initiator and validators should be able to add new reviewers after the workflow has started.

once done, rewrite selenium test parallelReviewAddReviewers-5.2.html

some code in JbpmActionsBean#getCanManageParticipants makes it possible to hide the add form once workflow is started in the mean time.

this should be possible until the workflow is ended (last task ended), by the initiator or any participant in the workflow.