Simplify some names
Put all arguments of Web_election.make in a record
Restructure Registration

 - define Registration.Site of type SITE_SERVICES
 - split election initialization in two parts:
   1. (inside Site) define a generic register_election
      function supposed to be callable from a running server
   2. (outside Site) register all elections that have
      been declared in the configuration file
 - main_election and featured election are no longer computed, but
   mutated dynamically
 - less things exported in WEB_SERVICES
Fix regression in logout handling on election home