Refactor handling of election pseudo-files
Prettier URLs for election pseudo-files
Add <rewrite-prefix> configuration option

The CAS protocol requires passing around absolute URLs, which cannot
be computed correctly by Eliom if it is behind a reverse proxy. This
option allows one to specify how to post-process URLs computed by
Eliom to match reverse-proxy configuration.
Booth: use URLs relative to the election

We assume the election URL is $PREFIX/elections/$UUID/election.json,
and we use URLs relative to $PREFIX elsewhere. For compatibility with
reverse-proxies.
Use relative path for booth parameter
Documentation for <rewrite-prefix>
Refer to OPAM 1.1.1 in opam-bootstrap.sh
Add a synthetic changelog