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

test/fixtures/enabled_modules.yml

@@ -59,3 +59,7 @@ enabled_modules_015:
   name: wiki
   project_id: 2
   id: 15
+enabled_modules_016: 
+  name: boards
+  project_id: 2
+  id: 16

test/fixtures/messages.yml

@@ -66,3 +66,14 @@ messages_006:
   author_id: 3
   parent_id: 4
   board_id: 1
+messages_007: 
+  created_on: <%= 2.days.ago.to_date.to_s(:db) %>
+  updated_on: <%= 2.days.ago.to_date.to_s(:db) %>
+  subject: 'Message on a private project'
+  id: 7
+  replies_count: 0
+  last_reply_id: 
+  content: "This is a private message"
+  author_id: 1
+  parent_id: 
+  board_id: 3