Move Rt crowd code into its own repository.

[seabbs]

The Rt crowd forecasting approach requires 4 steps.

1. Forecasts to be read in from a current Rt estimates
2. Expert forecasts to be made using crowdforecastr etc
3. Expert forecasts to be converted to samples
4. Samples to be converted into simulated cases and deaths.

The last two of these steps require custom code which will likely be used elsewhere. Might make sense to move this now as it carries many more dependencies than other tools used in this repository.