Use Eliom_service.static_dir instead of hard-coded paths
Use Eliom_service.static_dir instead of hard-coded paths

This lets Eliom compute proper relative paths and might help when
Belenios is running behind a reverse-proxy.

glondu/belenios,glondu/belenios,glondu/belenios,glondu/belenios,glondu/belenios