Clarify expand up/down arrows

[danvk]

Here's webdiff now:

"Show 228 more lines" is clear, but the up/down arrows are not. Here's pullrequest.com, by comparison:

Here's how Reviewable handles it. I find this extremely clear! Including a zero-line bar at the end of the file is clever.

Now that this is all in React, it should be easier to make the UI taller.