an obvious comment: we definitely need good indexing here.

we'll need to think a bit about the handling of current_revision_id, especially in the context of files/images. Each of those urls has a revision_id in it; we will need to make sure we keep that same id (could be in card_actions or card_changes). But we don't really want to have to do db lookups for those every time someone requests an image. We could keep the revision_id in the table, but I was looking forward to getting rid of that... perhaps an instance variable?