Replace non-functional images with CSS styled text

Replace non-functional images with CSS styled text

The plus-sign and minus-sign images, used only for showing more or fewer
copies, did not actually do anything when you clicked on them. In
addition, they can be reasonably represented as plain text + CSS, saving
an extra HTTP request per copy detail page that has more than 10 copies.

Moving the text + CSS into the <A> tag also makes them clickable, which
is kind of nice. And, being CSS, it's easy to change the colour scheme
to suit a different site.

Signed-off-by: Dan Scott <dan@coffeecode.net>