- Copy Button disabled or relabelled to reflect the selected style.
  - If no style is selected, the copy button is disabled.
  - Otherwise, the copy button's aria-label includes the style of the citation to be copied.
- Screen reader announcements for style selection, preview updates, and copy confirmation.

The changes here are from work done by @patdunlavey that was just going to be
constrained to the dgi base theme, but these fixes make more sense to live in
the public module that the citation block is created by so it isn't dependent
on any theme and can be more widely beneficial.