Add widget that shows underlying exchange rate for user selected display fiat currency

[pretyflaco]

This would give users transparency of the underlying exchange rate that is used for display currency.

The widget could live in the "Bitcoin Price" screen and show the exchange rate utilized for the user selected display currency.
(this is presuming that the Bitcoin Price graph will continue to display BTC/USD exchange rate and will not switch to BTC/User selected fiat currency)

Example:

User chooses Turkish Lira as display currency
Bitcoin and Dollar holdings are displayed in Turkish Lira equivalent, but there is currently no way to see what is the exchange rate that is used to calculate the equivalent.

In many high-inflation environments, like Turkey, exchange rates are closely followed and evaluated. Opaqueness can lead to mistrust. Showing the utilized exchange rate can mitigate by creating transparency.

[nicolasburtey]

I think we want to have btc/display currency in the price history screen. Not sure if that is possible @dolcalmi ?

[MaxwellDG]

@pretyflaco @nicolasburtey Still want this added? I could put a line above the graph with this info.

[nicolasburtey]

I think that would be useful to get the value as a scalar for now

Eventually we should get the graph also in the local currency (ie: btc/turkish lira instead of btc/usd) but we need to do work in the backend to generate those graph