Static on custom location

As an addition to #1834 config should be more aware of settings in omero.web

This PR automatically adjust Django STATIC_URL if FORCE_SCRIPT_NAME is set and propagate it while generating nginx config:

```
bin/omero web config nginx
bin/omero web config nginx --system
```

In order to test it please set the following in various configurations (that make sense of course) including empty values and check if matching config was generated:

example:
```
bin/omero config set omero.web.static_url
bin/omero config set omero.web.force_script_name '/omero'
```
or
```
bin/omero config set omero.web.static_url '/omero/static/'
bin/omero config set omero.web.force_script_name '/omero'
```
gives
```
server {
        ...
        location /omero/static {
        ...
        location /omero {
            ...
            fastcgi_split_path_info ^(/omero)(.*)$;
```