Comments interface for items promoted to Posts

[boonebgorges]

Currently, you can post a comment on an item during nomination #843, and you can interact with comments on the Nominated and Feed Item Reader interfaces. It's not currently possible to see these comments after an item is promoted to a Post. Let's improve this by expanding on the work in #843 to introduce a full-fledged comment interface when viewing a Post that was created through the PF workflow.

Note that PF items are duplicated as they move through the lifecycle. So, for example, pf_feed_item with ID 123 might become nomination with ID 124, which might become post with ID 125. Comments all have to live with the same item, so we'll have to use the 'id_for_comments' feature that exists elsewhere in PF.