Consistently add length 8 hash

[weaverryan]

Hashes (hash, chunkhash, contenthash) are used in 3 places (that I can think of):

1. The file-loader for images & fonts
2. The extract-text-plugin for CSS files
3. In the filename for the output js filenames

For case (1), we limit to 8 characters. For the other cases, we have no limit, which creates quite long filenames. It's minor, but I think we should default this to be consistently 8 long, which is plenty for uniqueness without being VERY long.

[stof]

Makes sense to me