Add current value page

Websites like CoinMarketCap load an atrocious amount of JavaScript and are simply too sluggish on mobile browsers. Pricewarp loads very quickly. The apps and even most of the sites are bad for privacy. It would be good to leverage the price data being collected for Pricewarp to show current prices.

We should implement the following features in a new "Prices" tab.

* Display Prices of favourites in table
* Show 24 hour difference if enough data is available
* Show 1 week difference if available

I believe showing a difference based on the current value and the daily average will more accurately reflect the state of play for cryptocurrency prices, as opposed to simply showing the difference between the current instant and the price exactly 24 hours before. This is what I believe CoinMarketCap does, and it can lead you to false conclusions about price.