Comments: Default avatars should be consistent

[aerych]

The comment list uses the mystery man icon for anonymous authors. On the other hand, the comment detail (a readery view) shows the default identicon. We should be consistent and pick one.

[kurzee]

Good one for the comments refresh.

[mattmiklic]

Let's use the gridicons-user icon like we do elsewhere. While identicons are used a lot on WordPress.com for unknown users, there's something just slightly dodgy looking about them, probably because they appear so often beside spam comments. I don't think we use identicons anywhere else in the app, so we can standardize on the user icon instead.