Use postedAt date for post time display, instead of createdAt

[Discordius]

I think the frontpage of greaterwrong right now uses the createdAt date for a post, but sorts by postedAt date. This makes it so that if someone had a post sitting around in their drafts for a while, the date shows as the time they first created the draft, and not the time they posted it.

[kronusaturn]

It already displays the postedAt date, but it's sorted by frontpageDate because it uses the 'frontpage-rss' view, and frontpageDate appears to reset to the current date whenever the post is edited. There doesn't seem to be a view available in GraphQL that returns frontpage posts sorted by postedAt.