Route calls to index.php via try_files with nginx

This makes short(er) sharing links even shorter, and would be neat for https://github.com/owncloud/core/tree/shorter_sharing_links

E.g.:
https://yourcloud.com/s/3pyaiaav7bt2i
instead of
https://yourcloud.com/index.php/s/3pyaiaav7bt2i
instead of
https://yourcloud.com/public.php?service=files&t=e91d871628e40fa23ac101a8e26805be
we have right now

@PVince81 
@josh4trunks