Change link_to_if_authorized to allow url paths. (Fixes #6195)

Both url paths (/issues/1234) and params hashes (:controller => 'issues')
are now supported by link_to_if_authorized.  The authorize_for method
requires a controller/action pair so urls need to be parsed against the
routes to find their controller/action.

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@4064 e93f8b46-1217-0410-a6f0-8f06a7374b81