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Though small, this code really does not belong to Election, which does
not (and should not) depend on a specific group implementation.

 - simplify WEB_BALLOT_BOX to inject a single credential at a time
 - in configuration file, replace <data> by <import>; <import> imports
   only elections (and their credentials) that are not already in the
   table (only UUID is checked)

 - base template is abstracted over login_box
 - per-election templates and services (except those related to
   update_credentials) refer to per-election auth services
 - login templates are parametrized over AUTH_SERVICES

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