US-2157: Round USD conversions

[rodrigoncalves]

As suggested by @TravellerOnTheRun, I decided to not only modify the existing formatTokenValues function, but split into 2 different functions (formatTokenValue and formatFiatValue) that each use a generic formatNumber function. This function expands the current implementation and now it should cover all different cases present in the application.

Below are a few screenshots showing the implementation in action. Please let me know if I missed anything.

[TravellerOnTheRun]

@rodrigoncalves
I've looked through the code and I don't like what I see:
First of all, the RBTC fee values will always show as 0.00 as it was before, because the value is toFixed(2). I already added the function which formatTokenValues which already contains the logic for showing the values in a relatively nice-looking manner. How about you edit that? We can discuss how many digits we want after 0 but let's make a single source of truth for formatting values?

[rodrigoncalves]

@rodrigoncalves I've looked through the code and I don't like what I see: First of all, the RBTC fee values will always show as 0.00 as it was before, because the value is toFixed(2). I already added the function which formatTokenValues which already contains the logic for showing the values in a relatively nice-looking manner. How about you edit that? We can discuss how many digits we want after 0 but let's make a single source of truth for formatting values?

Of course! That's why this PR is in draft, I'm still playing around with it. The formatTokenValues function seems to be a nice way to deal with token amounts due to different quantities of decimal places that we can have. However I didn't want to edit that function yet as all I need to do for a USD conversion is toFixed(2) and there you go. But I know for RBTC fee values cases (<$0.01), for example, it could be a problem.

[TravellerOnTheRun]

Some changes are still required for future proofing the app

[jormelCoin]

It should be fixed in this PR.

[jormelCoin]

I still see the NaN in Bitcoin transactions.

@jessgusclark, when you get a chance, could you, please take a look? Rodrigo says it works well for him but for me fails since I still see the NaN.

[jormelCoin]

I carried out the tests and it worked very well both iOS and Android.