Fixed bug with back button url not working when using uwsgi under sub-do...

The back button in FAB is hard-coded to be "/back" but if you are running FAB with UWSGI in a subdirectory (www.example.com/fab/) then you need the link to be to www.example.com/fab/back 

This patch uses url_for to generate the right link, so it detects if it is a subdirectory, etc, just like most other urls generated.