Change blocks on several slides

Change blocks on many slides in one step (and one version in the history). "edits" is a list of {"slide_id", "add", "update", "delete", "at_bottom", "base_version"}; on its slide each item adds new blocks ("add": [{"type", "props"}], on top or with "at_bottom": true under the others), changes the given props of blocks ("update": [{"id" | "ids": [...] | "inside": <card block id>, "props"}]) and deletes blocks ("delete": [ids]). Nothing is changed when one item is wrong (or its slide changed meanwhile). Returns the changed slides.