adding FORCE_SCRIPT_NAME to the settings.py close #11723

This allows to deploy omero on custom prefix using nginx. In order to test it you need to adjust your nginx.conf by adding the following:

```
    fastcgi_split_path_info ^(/omero)(.*)$;
    fastcgi_pass 0.0.0.0:4080;
    
    fastcgi_param PATH_INFO $fastcgi_path_info;
    fastcgi_param SCRIPT_INFO $fastcgi_script_name;
    # fastcgi_param PATH_INFO $fastcgi_script_name;
```
and setting ```$OMERO_HOME/bin/omero config set omero.web.force_script_name '/omero'```

see https://trac.openmicroscopy.org.uk/ome/ticket/11723#comment:3