Style links to attachment preview
CSVPreview guesses character encoding

The CSV library will chock unless you get the encoding right. We can
use the charlock_holmes gem to guess the encoding of the file.
Handle CSV preview encoding errors

This should be a rare occurance, but if we cannot guess the encoding
of a file, or if we get it wrong, we want to gracefully continue.
CsvPreview reports if the preview is truncated
Add white-links mixin
Style attachments preview page
Simplify character encoding checks in CsvPreview

Using a simpler form of encoding checking based on @alext's work on
the imminence project.
Correct selector in feature step