Adds methods for loading and adding settings.

git-svn-id: http://svn.redmine.org/redmine/trunk@13719 e93f8b46-1217-0410-a6f0-8f06a7374b81
Skip uniqueness validation when updating a setting without changing its name.

git-svn-id: http://svn.redmine.org/redmine/trunk@13720 e93f8b46-1217-0410-a6f0-8f06a7374b81
Specify the order for finding a setting.

git-svn-id: http://svn.redmine.org/redmine/trunk@13721 e93f8b46-1217-0410-a6f0-8f06a7374b81