Avoids duplicate work, and is a step towards allowing the next
states to be modified by third-party code.
Also:
- fixed incorrect underlined count, due to reviews being labeled as
learning cards
- fixed reviewer not refreshing when undoing a test review, by splitting
up backend queue rebuilding from frontend reviewer refresh
- moved answering into a CollectionOp
The explicit flush was clearing undo history, and the hook will need
re-working to support propagating OpChanges correctly. It will likely
come back as a GUI hook, instead of one in pylib.
