Provide own arrow images

Copied from mediawiki/core's skins/common as of
1683741e7177eeff63e4653f4a4a633a90c9cf64.

This is the best fix I'm willing to make at 2 AM, and, frankly, the
best fix I'm willing to make to LiquidThreads at all.

Bug: 70412
Change-Id: I307d82363fc81364cdbc9c9ee3f990ff326538e9