Require explicit listing of published elections in data dirs

Do no longer rely on the presence of result.json to decide whether an
election should be imported or not. Now, directories listed in <data>
tags must contain an "index.json" file that lists sub-directories that
should be imported by the web server.

Additional changes:
 - allow several <data> tags
 - small memory optimization in election.json parsing
Move demo password_db.csv outside of election data dir
Rename election_update_credential{ => _post}
Rename election_update_credential{_form => } and move it one level up
Remove almost useless Services.preapply_uuid
Move ballot reference to VOTING_SERVICES
Merge Services into Web_common
Move election_file param declaration to Web_common, for consistency