Invert persistent vs ephemeral cell IDs

[sourishkrout]

Sort of revert what we did in this PR: #938

And then salvaged in this PR: #1252

The remaining problem right now is that when cell IDs are turned off we have to remove them from the document. By having the core logic rely on the ephemeral IDs and make them the same as the persistent ones (already the case). Cell IDs will be optional again no matter whether they are persistently present or not.

[sourishkrout]

When identity is turned off for cells, saving this file should write exactly the same, as in don't add or remove any IDs from cells.

In the current implementation it will either add IDs to all cells or remove them from all cells.