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In each directory, always open in the following order:
 1. third-party libraries
 2. serializable
 3. signatures
 4. common

Also, always open modules from lib/* before. This order is reflected
in *.mllib files.

Additionally, open more frequently serialization modules.

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_".