Add handshake key pairs to DB, remove inactive contacts

This branch adds optional handshake key pairs to contacts and the local identity. A future part of #1232 (closed) will generate and store handshake keys for new and existing identities. Handshake keys for existing contacts will eventually be exchanged via a new client, but not as part of #1232 (closed).

Handshake keys for contacts are stored in the Contact class rather than the Author class, as they're distributed separately from the author structure.

Since I was modifying the Contact class and the DB schema, I took the opportunity to remove the unused 'active' flag, fixing #1276 (closed).

Part of #1232 (closed), fixes #1276 (closed).