Hide 'Scan' option for already DRM-Free books

This is in two separate commits because the first commit introduced a problem and the second commit fixes it.

I thought it would be nice to just not show the "Scan" button when books are already DRM-Free, because it's faster to just download the PDF or CBZ already provided. Adding that was relatively simple, however my implementation may be somewhat brittle to DOM changes.

This introduced a problem which is that the "Setup" option would not always display if the first book was DRM-Free. With this change since the "Scannable" books are sprinkled sporadically throughout, at least in my personal comics, I figured that just having ANY "Scan" button start as a "Setup" button made sense. That way any "Scannable" book that I encounter I can use to go through the "setup" process instead of trying to find the first one. This also made the "Unusable" option obsolete, so I removed it.

I'm not sure if this is a desirable feature for anyone else, I just thought it might be nice.