see the following for background discussion:
http://groups.google.com/group/ankisrs-users/browse_thread/thread/4db5e82f7dff74fb
- change sched index to the more efficient gid, queue, due
- drop the dynamic index support. as there's no no q/a cache anymore, it's
cheap enough to hit the cards table directly, and we can't use the index in
its new form.
- drop order by clauses (see todo)
- ensure there's always an active group. if users want to study all groups at
once, they need to create a top level group. we do this because otherwise
the 'top level group' that's active when everything is selected is not
clear.
to do:
- new cards will appear in gid order, but the gid numbers don't reflect
alphabetical sorting. we need to change the scheduling code so that it steps
through each group in turn
- likewise for the learn queue
because group deletions are likely to be a semi-common operation (esp. for new users trying out shared material), deleting groups will no longer cause a full sync. in order to avoid syncing issues, we now allow cards/facts/etc to point to an invalid group, and in that case, we just treat them like they're in the default group
