Make users finish the tutorial

Make users finish the tutorial, not just start it (esp. if they reload the tutorial, or leaves and comes back before finishing it)

The fix is to separate the URL used to start working on an app (the one linked from "Start Contributing Now!" on the app page), from the one linked to at the end of a tutorial.

This fix is backwards compatible - the URL used in tutorials, appname/newtask hasn't changed. However, this  URL now sets the cookie unconditionally.

"Start Contributing Now!" now leads to appname/presenter, which redirects either to appname/tutorial if there is one and the cookie isn't set, or otherwise to appname/newtask

Note that neither appname/tutorial nor appname/presenter doesn't set the cookie.