Display watch link only when inside project with logged-in user (#2549).

git-svn-id: http://svn.redmine.org/redmine/trunk@12868 e93f8b46-1217-0410-a6f0-8f06a7374b81
Makes #watcher_link return an empty string when given nil.

git-svn-id: http://svn.redmine.org/redmine/trunk@12869 e93f8b46-1217-0410-a6f0-8f06a7374b81