Detect when a post has been edited and repost it (in some cases)

The feed system currently will read each item once and post it to the room. However, it does not account for items which might change. Ideally if there is a change, we should repost the item again. Perhaps looking at the pubDate might help us.

Start by tracing the feed system's item-reading and room-posting flow, then check how pubDate is currently handled. Done means changed items are reposted when appropriate while unchanged items are not duplicated.

Written by the indexing model from the issue text.