Add named routes for attachments and use route helpers in #link_to_attachment.

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@11187 e93f8b46-1217-0410-a6f0-8f06a7374b81
Adds a named route for thumbnails and use route helper in #thumbnail_tag.

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@11188 e93f8b46-1217-0410-a6f0-8f06a7374b81