Prepares the chat's @everyone: it adds chat_messages.mentions_everyone,
and frees the name everyone, which no account may have any more.
f
up(db: DatabaseSync): void
Run once by runMigrations. The messages that a moderator or an admin
already sent with @everyone get the flag, so that editing one later
doesn't notify everyone of an old message; none is notified now. A user
named everyone, in any case, is renamed to the first free everyone_2,
everyone_3…, with no alias left behind, and an alias everyone is
dropped. Both are logged.
Usage
import * as mod from "api/db/migrations/0023_mention_everyone.ts";