Genericity : remove legacy SILL config

This is a list of hard written linked to code.gouv.fr or SILL related config.

- [x] In `web/index.html` inside the `<head>` it contains a lot of variable linked to gouv.fr and SILL (tracker, Twitter, Facebook)
- [x] There is a lot of variable with the name SILL inside.
- [ ] Inside the DB some variables have the name of a source : `SILLid`, etc