Add paperfold left edge to inlined replies

I'm liking the effect. It's distinct from the flat aesthetic that's
popular nowdays, while still being less gaudy than mid-2000s GRADIENTS,
SPIKY BADGES, "BETA", 45 DEGREE STRIPES, and LOGO REFLECTIONS.

I guess I'm still guilty of the subtle texture pattern and the
low-contrast text, but to be fair it's not the body text, only the
postinfo.