Rekindly 0.3.1

Changed

• Safe updates: before an update changes the database, the app saves a copy of it
  to /data/backups/pre-update-….sqlite (the newest 3 are kept). Without a copy, it
  does not update. The update itself runs with foreign keys switched off, so rebuilding
  a table can never take dependent rows (entries, facts, links, …) with it; afterwards
  every reference is checked, and the app stops with a clear message instead of running
  on a damaged database. See deployment.

Upgrading from 0.3.0

Nothing to do: 0.3.1 has no database migrations. From the next update that changes
the database on, the copy is saved first.