add "host" parameter

Close #52 (closed)

The directmairie.hosting.host parameter of the DirectMairie application must be configured, as it is used for the emails sent by the application.

see: /backend/src/main/resources/application.yml

directmairie:
  database:
    host-and-port: localhost:5432
  pictures:
    root-directory: /tmp/directmairie-pictures
  hosting:
    host: directmairie.adullact.org
  mail:
    from: 'no-reply@${directmairie.hosting.host}'