feat(web): revise undocumented meta.json endpoint

[robherley]

There's a secret /meta.json endpoint that hasn't been updated.

This cleans up the structure a bit and adds new fields:

{
  "commit_sha": "6c30767efc2acb9c5c76edfb8e314c96a36cc7d2",
  "endpoints": {
    "http": "http://localhost:8080",
    "ssh": "ssh://localhost:2222"
  },
  "guesser_enabled": true,
  "limits": {
    "file_size": {
      "bytes": 1048576,
      "human": "1.0 MB"
    },
    "files_per_user": 100,
    "session_duration": {
      "human": "15m0s",
      "seconds": 900
    }
  }
}