Goal: Reach Feature Parity with Dark-Classic, and Migrate User Canvases

[StachuDotNet]

This issue aims to

• cover the general topic of "when can users port their canvases from dark-classic to dark-next?"
• and outline the migration process, of our users' code from -classic to -next.

We're a few months away from migrating any user code to -next, but here's a rough sketch in the meantime:

• bring back cloud runtime
• secrets
• static assets
• http handlers
• user dbs
• decent editor
• crons
• workers
• announce various migrations - blog, email to users w/ running canvases, etc
• code to migrate stuff
• opt-in phase
• migrate the rest
• shutdown-classic