changeSlug(): void
Gives item the slug slug, keeping its current one as an alias. An alias
always names the item itself, so redirects never chain; taking back one of
the item's own aliases swaps it with the current slug. Unless exempt (a
moderator), an item's slug changes at most RENAME_LIMITS.SLUG.count times
in its window, counted from its aliases. A slug equal to the current one
changes nothing.