Non-monolithic project configuration

[nomeata]

For the live.lean-lang.org instance, also due to its nix setup, managing the set of offered projects is somewhat tedious.

It would make it easier if

• the server component would take an environment variable LEAN4WEB_PROJECT_DIRECTORY to point at the directory that contains all the projects.
• there would be no config.tsx file. instead upon server statup, it would enumerate the directories in LEAN4WEB_PROJECT_DIRECTORY.
• a project file could have a lean4web.js file (or similar format) to set
  • the name shown in the drop-down
  • opt-out of being shown there (for projects that we want to host but not advertise)
  • other settings like example files.