Auto-Update ETH & DAI price

[theschweitz]

We currently have to Update ETH & DAI price manually on the backend - this requires a daily or twice-daily update for the user on the backend. It was initially intentional not to use external APIs/data for this plugin, but in the effort to make it easy for organizers, updating daily (or hourly in cases of extreme volatility) is not ideal.

Perhaps there's a way we can source price data from multiple endpoints.

Also to be taken into consideration: This fix would need to read Event Currency data from Pretix, and input price as Event Currency.

[mikulas-mrva]

Most likely we would use the MakerDAO for USD, there is a ChainLink oracle for EUR to be investigated.