Chat notifications were blindly calling review.get_absolute_url() to

get the URL to link to for the notification. Unfortunately, for

review replies, this doesn't work because we don't put an anchor for

those into the page.
This change fixes it so if we have a comment (which we use for the

notification text), we open directly to that comment, and if we don't,

we link to the parent review.

Ran unit tests.
Verified that generated links worked correctly.