Separate out objective change in request permission update.

Per [72813220](https://www.pivotaltracker.com/story/show/72813220).  Neither branch with permission update was reachable with no change in objective.  This retains the logic for objective changes but adds a special case for when there is no objective change. This works to ensure the new Assignee has write permission.

Note: the logic regarding Program/Audit roles is more complex, and we may need to revisit how assignees keep their permission to the request folder after being reassigned off of it.