Added an undelete thread option in routing, controller, and layout

This pull request adds the option for users to undelete a thread. When the user visits his deleted threads  page, they no longer see the action 'delete' (which makes no sense at all) but now they see the action  'undelete'.

The code for undeleting a thread was already there. All I did was adding a route, a controller, and some updates to the standard templates...