CoursePrice.price should be a decimal value

[singingwolfboy]

Currently, the course price API serializes the price decimal field as a floating-point number, potentially losing precision. As a result, the CoursePrice flow type defines the price field to be a floating point number, rather than a decimal.

In order to maintain precision, the course price API should serialize the price field as a string, and the front-end should parse it into a decimal.js-light instance. This is the same technique we use for the amount field in the the coupon API.