- fetch sfld and csum when fetching notes, to make it cheaper
to write them back out unmodified
- make `fields` private, and access it via accessors, so we can
still catch when fields have been mutated without calling
prepare_for_update()
- fix python importing code passing a string in as the checksum
- Rework V2 upgrade so that it no longer resets cards in learning,
or empties filtered decks.
- V1 users will receive a message at the top of the deck list
encouraging them to upgrade, and they can upgrade directly from that
screen.
- The setting in the preferences screen has been removed, so users
will need to use an older Anki version if they wish to switch back to
V1.
- Prevent V2 exports with scheduling from being importable into a V1
collection - the code was previously allowing this when it shouldn't
have been.
- New collections still default to v1 at the moment.
Also add helper to get map of decks and deck configs, as there were
a few places in the codebase where that was required.
- notes with wrong field count are now recovered instead of
being deleted
- notes with missing note types are now recovered
- notes with missing cards are now recovered
- recover_missing_deck() still needs implementing
- checks required
