Adds watch/unwatch functionality at forum topic level (#1912).

Users who create/reply a topic are automatically added as watchers but are now able to unwatch the topic.

git-svn-id: http://redmine.rubyforge.org/svn/trunk@1878 e93f8b46-1217-0410-a6f0-8f06a7374b81

test/fixtures/watchers.yml

@@ -3,4 +3,8 @@ watchers_001:
   watchable_type: Issue
   watchable_id: 2
   user_id: 3
+watchers_002: 
+  watchable_type: Message
+  watchable_id: 1
+  user_id: 1