SQLite schema versioning and migrations

[singular0]

History creates its table with CREATE TABLE IF NOT EXISTS and has no schema version, so there is
no way to change the history model on an existing install without guessing at what is already there.

Reference: src/model/history.cpp:190.

Tasks:

• Record a schema version in the database.
• Add a migration path, and decide what happens when a database is newer than the binary.
• Land this before the conversation model changes, not after.

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Land before #30.