there were a number of issues with preloading:
- it could result in duplicate IDs in the document
- embedded styles failed to apply, because a preloaded item was
overriding them
- the type answer code needed rewriting to support it
so we're back to something closer to the old approach - a single node
that we fade out, update, and then fade in again. this means there's a
longer delay when revealing mathjax, but should minimize the breakage of
existing notes
- to minimize pop-in of images and mathjax, fade the previous card
out and fade the next card in
- render the answer at the same time as the question so it can be
shown immediately
- move reviewer css to separate file
- remove image preloading code that should no longer be necessary
