updateDump(): Promise<Dump>
Applies request to a dump, for PUT /api/dumps/:dumpId. A new link has its
preview fetched and gives the dump its page's title, unless request sets
one. A file dump ignores url. A new slug goes through changeSlug,
which exempt (a moderator) frees from its rate limit; a new title leaves
the slug as it is. Clients get the dump as dump_updated, or a
dump_deleted when it turns private and a dump_created when it turns
public. Anyone @mentioned in the note is notified, and when it turns public,
its followers are as when it is posted (notifyIfWentPublic).
Promise<Dump>