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Move code specific to a group implementation there, and use it
everywhere else.

They do not behave as expected on serialized JSON objects. Instead,
directly export a reader and a writer as needed.

The "p" property is specific to finite fields anyway, so there is no
gain in abstracting the group element type in it.

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