Replace ApiServer with `dark-editor` canvas

[StachuDotNet]

In other words: rewrite ApiServer in Dark

Useful links:

• https://github.com/darklang/dark/blob/main/fsharp-backend/src/ApiServer
• https://darklang.com/a/dark-editor

[StachuDotNet]

Thinking out loud, some TODOs:

• [ ] review existing functionality within ApiServer
• [ ] review existing testing of ApiServer
• [ ] review internal functions (esp. the recently-exposed ones)
• [ ] write out the existing endpoints
• [ ] spend a few minutes sketching out what's needed for each of them
• [ ] spend some time thinking through how the ApiServer testing can be replaced/improved in Dark
• [ ] outline some rough ordering
• [ ] review current state of dark-editor
• [ ] review current state of other dark- canvases and ops-
  • [ ] note which are appropriate to eventually be merged to dark-editor
• [ ] decide how to host ui.html and login.html
  • [ ] ? strip down ApiServer to just host ui.html? (easiest)
  • [ ] ? push to static assets during CI? (sounds cool)
    • [ ] (how fast are static assets reliably available)
  • [ ] host via some other backend server? (enh)
• [ ] update documentation / references to ApiServer
• [ ] write a blog post

[pbiggar]

Relevant but out of date from Darklang-next