Move type declarations from Web_election to Web_signatures
Changes in saved_service handling

 - store the service itself
 - move closer to site services
Move ELECTION_BUNDLE to Web_signatures and drop BALLOT_BOX_BUNDLE
Changes in metadata handling

 - metadata is specific to web, move it there
 - create Web_serializable, and also move randomness there
 - metadata is not optional (but its components can be)
 - rename user_type into user_domain
 - serializable version of ACLs
Simplification of the web_election type

Now, everything needed by the web server to handle an election is
bundled into a module of type WEB_ELECTION which is passed around
everywhere.
Add metadata for demo election (ACLs are now mandatory)
Remove dead code
Remove useless metadata generation from demo script
Remove hard-coded default group
Always check finite field parameters
Move EMPTY to Signatures