For now, they are not referenced anywhere, but seem to work when
accessing directly their URL.

Rationale: we don't want templates to be able to call them.

SITE_SERVICES.cont is now an Eliom reference that can be directly used
wherever a continuation is expected.

Incidentally, if a user is already logged in, log out first.

Keep "Auth_" prefix for auth systems.

 - rename AUTH_INSTANCE into AUTH_HANDLERS
 - add "logout" to it
 - rename existing "handler" into "login"
 - logged_user contains AUTH_HANDLER instead of CONT_SERVICE

All modules in src/web start by either "web_" or "auth_".

 - 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

Summary:
 - WEB_ELECTION gets a new submodule of type ELECTION_SERVICES
 - per-election services do no longer take UUID as a param, but are
   registered at a path containing it
 - no more ESIndex, election home service is back
 - inline some submodules of SITE_SERVICES, drop ALL_SERVICES
 - rename old SITE_SERVICES into CORE_SERVICES
 - move election-specific templates to a new ELECTION_TEMPLATES; the
   templates in the new signature do not longer take an "election"
   argument but TEMPLATES.Election is a functor that takes a
   WEB_ELECTION and returns an ELECTION_TEMPLATES
 - adapt the booth to the new path layout

Additional changes:
 - merge VOTING_SERVICES into ELECTION_SERVICES
 - inline {SITE,AUTH}_TEMPLATES into TEMPLATES
 - all templates take a final () argument to emphasize their impurity

Each election may have an owner who can perform administrative tasks.

The source code is public now...

 - Eliom_config.(use get_config instead of parse_config)
 - presence of <source> is no longer technically enforced
 - simplify configuration of auth systems
 - auth systems directly register themselves

The new field is used as root for services specific to an auth system.

Do no longer rely on the presence of result.json to decide whether an
election should be imported or not. Now, directories listed in <data>
tags must contain an "index.json" file that lists sub-directories that
should be imported by the web server.

Additional changes:
 - allow several <data> tags
 - small memory optimization in election.json parsing