Gives each user settings stored on the server, so that they follow the
account across browsers and the server can read them: the digests need the
reader's choice to receive them and their style. user_settings holds one
row per user who changed anything; a user without one has the defaults.
f
up(db: DatabaseSync): void
Run once by runMigrations. Existing users get no row, so they start with
the defaults, digests on, like a new account.
Usage
import * as mod from "api/db/migrations/0019_user_settings.ts";