Ability to uncheck "Multiple values" for existing custom fields (#12251).

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@11167 e93f8b46-1217-0410-a6f0-8f06a7374b81
Adds a message about disabling multiple values on custom field form (#12251).

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@11168 e93f8b46-1217-0410-a6f0-8f06a7374b81
Copyright for 2013 (#12788).

Patch by Daniel Felix.

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@11169 e93f8b46-1217-0410-a6f0-8f06a7374b81