Highlight new comments in post view and show how many there are in list view

[BoringBuffalo]

If we keep track of when users last checked a post's comments, we can

1) highlight comments that were posted after the last time they checked

2) in the list view, change the link to the comments to say something like '97 Comments (96 New)'

[Polsaker]

This could be done together with #225 and a view counter for posts. We'll have to start tracking visits (uid, date) to posts.