Don't let the user pick non-unicode emoji

[nmeierpolys]

There are a few non-unicode emoji included in the picker component that don't work well when outputting to unicode. They're visible in the picker because there are image files associated with them, but they don't have unicode versions. Right now, if the user picks one of these, it inserts the name of the emoji with colons on either side.

There are 7 emoji with this issue (out of ~920):

winky_face
wow
neckbeard
metal
fu
black_square
white_square