Jormungandr: add param to force https in link

for navitia.io to work behind reverse proxy we need to be able to force
https scheme in flask link (url_for).

To force this add the param
```
# Behind reverse proxy we might want to force https for the jormungandr
link
# default is False
FORCE_HTTPS = True
```

in the settings file.
It would be easier to name it ```url_for``` so  we just have to change the import and not all the call.