Clean up story-map JSON

[leopiccionia]

Remove layers metadata from story-maps block attributes.

Currently, story-maps editor save to database, for each story-map, metadata related to all layers, even though it's mostly unused and could be retrieved during runtime on editor (it doesn't seem to be required for rendering a story map on front-end). It's wasteful and a huge memory burden.

See old branch for previous work on this front (it's lacking deprecating the old block and creating an automatic migration to new block format).