Add attribution to APIs we use

We should be good citizens. Maybe somewhere accessible from Settings?

E.g.

CoinGecko: https://www.coingecko.com/en/api_terms

> You undertake that in your usage of the CoinGecko API, you shall duly attribute ownership of the CoinGecko API to CoinGecko by displaying prominently the message “Powered by CoinGecko” in a legible font (an example of a legible font type being “Arial”) no smaller than font size 10. Further guidelines on the attribution of ownership to CoinGecko and usage of the CoinGecko Brand and can be found at https://www.coingecko.com/en/branding (“Brand Usage Guide”).

Etherscan: https://goto.etherscan.com/apis

> Source attribution via a link back or mention that your app is "Powered by Etherscan.io APIs" is required except for personal/private usage.