[NT-411, NT-429] Show total amount currency conversion label

[ifbarrera]

📲 What

Shows a currency conversion label beneath the Total amount in the PledgeSummaryViewController if the user's preferred currency is different from the project's currency. If the user does not have a preferred currency set, we use USD to determine whether a conversion label is needed. The conversion label will read About $10.50 and display the converted amount to 2 precision points.

I also removed the + icon from in front of the total amount as part of NT-429.

🤔 Why

To give users a better idea of how much they are pledging, if the project's currency is different from their preferred currency.

🛠 How

• Added roundingMode, maximumFractionDigits and minimumFractionDigits parameters to our Format.currency function so we can customize these parameters when we format the converted amount.
• added a new totalConversionLabel to PledgeSummaryViewController

👀 See

Trello, screenshots, external resources?

Pledge	Update	Logged Out

♿️ Accessibility

• Tap targets use minimum of 44x44 pts dimensions
• Works with VoiceOver
• Supports Dynamic Type

🏎 Performance

N/A

✅ Acceptance criteria

• Navigate to a project with the same currency as your preferred currency. Select any reward. You should not see a conversion label on the pledge screen.
• Navigate to a project with a different currency than your preferred currency. Select any reward. You should see a conversion label below the total on the pledge screen.
• Log out of your account. Then, navigate to a project with a currency that is not USD. Select any reward. You should see a conversion label below the total on the pledge screen.

[cdolm92]

Minor alphabetizing suggestions