Web-based configuration file editor and previewer

- [ ] endpoint and page that shows `advancedJsonPreview` ([see live](https://github.com/icpc/live-v3/blob/ecfac8543b560b88362ba3e28e878aadb80f8a47/src/backend/src/main/kotlin/org/icpclive/admin/Routing.kt#L81))
- [ ] page to edit `advanced.json` and `custom-fields.csv`