Replace nsfw.png with a neutral image, per issue #90.

This is a much more neutral image for nsfw.png, which should deal with the concerns raised in #90 (closed) with the current default.

mentioned in issue #90 (closed)

The replacement isn't going to be readable on a dark background. Other than that, no complaints.

closed

@normandy: FWIW, it's outlined by a 1 px white outline specifically to be readable on a dark background.

But, based on the discussion in #90 (closed), it looks like the solution is just going to be to use text, rather than an image, by default.