Instagram backfeed: handle nested comments

[snarfed]

on IG, you can reply to comments to create nested comments. example: https://www.instagram.com/p/CCcSqRspIW4/ . it looks like we don't capture and backfeed those right now, but they're in the JSON we scrape, so we should!

[snarfed]

working now! examples: https://snarfed.org/2020-07-09_41761#comment-2775818

...at least for discover. poll is still tricky, since it looks like the comment count we use to trigger fetching the individual photo (with comments) only includes top level comments, not nested comments...? hrmph.

[snarfed]

ah, no, the comment count already included the nested comments, so we didn't think anything had changed, so we didn't fetch the comments. ok, working then.