The signature is split into:
 - SITE_SERVICES: site-wide services, with single instances
 - ELECTION_SERVICES: per-unarchived-election services
 - VOTING_SERVICES: per-open-election services

For the moment, per-election services are still site-wide with single
instances.

Other changes:
 - the "global continuation" getter is renamed to "cont", so that its
   container module can be directly cast into CONT_SERVICE
 - move remaining service declarations out of Services
 - move service registration code close to service declarations,
   following the same Services/Register pattern as in Auth_common
 - various simplifications in Registration