Use form actions rather than .json endpoints to create/save playground apps

[Rich-Harris]

No description provided.

[dummdidumm]

What do you think will get better from this? The progressive enhancement argument is irrelevant here, because the repl won't load without JS

[Rich-Harris]

just think it'd be nicer to have the actions colocated with the loaders. admittedly there'd be slightly more wrangling on the front end to marshal the data