NOTE !!This change will require you to start building from a new folder!!

This is the first step in a change to cleanup the UI & move the non-ui
code into an API layer that can be tested/hosted from a new GUI.

- Build Sonic Pi from the app folder
- Fix up all scripts, installers, CI to use that folder
- new layout:
    app/
        external
        server
        gui
- Add an empty api module, static linked to the main app (this will get
non-ui interface code)
- Fix up documentation