Make read status more granular: on a message basis rather than conversation level

Currently, viewing a conversation marks the whole thing as being read, even if you didn't scroll up to see all the messages in the convo. This confuses some users when they read the most recent message in a long conversation, and the next time they view the conversation all the older messages have collapsed to a preview.

If individual messages had an unread status, and they didn't change to read until scrolling into the viewport, it would be easier to keep track of which messages you've read.