added 'Unenroll User' action to the message campaign channel

https://trello.com/c/ATTHwN1t

Added the 'Unenroll user' event as an channel action in the tasks channel
definition in the message campaign module.

Set 'ReferanceTime' and 'StartTime' keys in the 'Enroll User' action as
non-required fields

Change-Id: Id1a8a839e5a54d48a4199cf52ac11ece12c53566