Adds an option to send email on "Assignee updated" in application settings (#16362).

git-svn-id: http://svn.redmine.org/redmine/trunk@12975 e93f8b46-1217-0410-a6f0-8f06a7374b81
Wrong argument.

git-svn-id: http://svn.redmine.org/redmine/trunk@12976 e93f8b46-1217-0410-a6f0-8f06a7374b81