Errors when user inputs two cards in a row without creating new span

[will-0]

Currently, the anki note editor allows you to submit a new card, even if one has already been submitted for the last-created span added to the text. In theory, it should only be able to add new cards if the order goes "new unverified-anki span → note added". It should not be possible to do "new unverified-anki span → note added → note added". Consider using a global "ready" flag that is set when a new unverified-anki span is created, and reset upon note addition.