DirPortFrontPage should serve the 'tor-exit-notice' static page

DirPortFrontPage should serve the 'tor-exit-notice' static page rather than tor's man page. Check https://leap.se/code/issues/5241 . This is also related to https://github.com/irregulator/leap_platform/commit/8d6b36eff5fcdd39bf1065eb481a0f98a6135696