Stop nesting the .comment-thread divs when too deep

Browsers will stop applying visual styles to elements that are very
deeply nested. Our .comment-thread class wraps around each comment +
its replies (instead of each comment), but unfortunately, browsers
aren't very happy with that on long nested threads, and things look
broken.

(In firefox, links lose styling and everything becomes a pile of text at
the bottom. In chrome, the comments are spaced apart, but lose
indentation).

So since we know approximately when this happens, let's stop nesting
before we reach that point. Early in the thread, .comment-thread will
wrap around the comment and its replies. At 180 replies in,
.comment-thread will act like the .comment-wrapper (avoiding further
nesting)

For #726.