Correctly display currencies

[mindgraffiti]

See discussion / duplicated from: woocommerce/woocommerce-android#148

To correctly display the currency data we get from WooCommerce orders, we'll need to fetch the site currency config from the settings endpoint. The data needed is:

• Currency symbol
• Whether to display the currency before or after the value
• Whether to add a space between the currency and the value
• How many decimal points to display

Corresponding screen in WooCommerce settings:

If an order is made in one currency and the site currency changes after that (or if a multi-currency extension is used), I'm not sure what we should do - probably a best guess approach will have to do.

[bummytime]

When working on this issue, please verify #366 is also fixed.

[bummytime]

A couple of notes around currency settings:

1. It appears like the top earners and order stats endpoints update the returned JSON's currency code to whatever site settings has immediately after changing it (for any date range)

2. The orders endpoints will update the returned JSON's currency code for any new orders placed after the site settings change (the older orders will "remember" the previous currency code)

3. For the returned JSON from the orders endpoints, the site currency settings for woocommerce_price_num_decimals seems to be used immediately across all of the orders regardless of when this currency setting changed (e.g. "total": "253132880.000")

4. The woocommerce_currency_pos, woocommerce_price_thousand_sep, and woocommerce_price_decimal_sep values in site settings are not used in any endpoint. (and probably shouldn't be?)

So given this ☝️ , I am proposing the following:

1. For anything orders, we should probably respect the endpoint's returned currency code JSON values

2. For stats we can default to the site settings currency code (since the endpoint's values change immediately to that anyway)

3. For any displayed currency value that is not a "friendly" value (e.g. $123.34), let's use the following site setting currency values when formatting: woocommerce_price_num_decimals (number of decimal places), woocommerce_currency_pos (symbol on left or right), woocommerce_price_thousand_sep (thousands separator), and woocommerce_price_decimal_sep (decimal separator)

4. For any displayed currency value that is a "friendly" value (e.g. $1.6k), let's use the following site setting currency values when formatting: woocommerce_currency_pos (symbol on left or right)