Add module Platform: single interface to functions that depend on the
runtime. For now, the native (existing one), and a dummy one that
constitutes a skeleton for the js one.

All calls to Cryptokit, Z, Calendar (which depends on Unix) in the
command-line tool now go through this module.

When the login service is used for the first time, it asks for a
password database. For now, it is the administrator's responsibility
to do it before advertising the election.

 - 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

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

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

 - 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.

There were 3 phases: configuration parsing, service definition and
service registration. The last two can be merged, since there is no
direct external reference to the service of a specific auth system. As
a consequence, there is no need for a Register sub-fonctor in
AUTH_INSTANCE: service definition and registration occur directly in
the Make function, called by instantiate.

Summary:
 - move rewrite_prefix to Web_common
 - each auth system now has an $init function that registers its
   configuration spec and an $exec function to Auth_common
 - an auth system may support multiple instances via configuration;
   user_type is specific to each instance
 - when parsing is done, each $exec is called with an $instantiate
   callback; $exec is supposed to call $instantiate for each instance

TODO:
 - default fallback when there are several instances
 - generic login should do security_log
 - auth system instances should not set user_type themselves
 - it should not be possible to login twice
 - admin login
 - update doc

As in "continuation". This is more generic than just logging out...

Auth systems are wrapped into a module (with signature AUTH_SYSTEM) to
cope with polymorphism.

This revision should be bisimilar to the previous one.

Summary:
 - each auth system registers $name and $service
 - $service stores a logout handler in Auth_common.user
 - the generic login service redirects to $service if its parameter
   matches $name
 - the generic logout service clears out Auth_common.user and invokes
   the stored logout handler
 - the default auth system is the one registered last
 - Templates directly query Auth_common for available auth systems

TODO:
 - enforce user_type = $name (currently not the case for CAS)
 - export Auth_common.register_auth_system
 - move existing auth systems out of Auth_common
 - integrate with config parsing
 - rework CAS
 - generic login should do security_log
 - do admin login

Currently, it takes the auth_system name as an optional parameter, and
redirects to the existing services. This revision should have the same
behaviour as the previous one, except for login links and the
associated additional redirections.

Summary:
 - Auth_common.Make is now split in two functors: one for declaring
   services (Make) and one for registering them (Register)
 - auth_systems moved to its own signature
 - login and logout are in their own signature
 - new signature ALL_SERVICES

This is preliminary work to make the authentication system more
flexible. This big refactoring should be semantically equivalent to
the previous commit.

Summary:
 - create Auth_common and Web_signatures
 - split Web_election out of Web_common (to cut dep cycle)
 - move service definitions out of Services
 - functorize Templates