Makes user unwatch what he can no longer view after its permissions have changed (#3589).

A rake task (redmine:watchers:prune) is also added to prune existing watchers.

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@3167 e93f8b46-1217-0410-a6f0-8f06a7374b81

app/models/message.rb

@@ -42,6 +42,10 @@ class Message < ActiveRecord::Base
   
   after_create :add_author_as_watcher
   
+  def visible?(user=User.current)
+    !user.nil? && user.allowed_to?(:view_messages, project)
+  end
+  
   def validate_on_create
     # Can not reply to a locked topic
     errors.add_to_base 'Topic is locked' if root.locked? && self != root