[Open edX Platform] Add subscription and membership models for course libraries

**Opportunity Objective** Currently, the primary model for users to access paid courses is by paying for individual courses or a program. Building eCommerce functionality that would allow the users limited or full access to the course library for a subscription fee can give Open edX platform users more options for setting up an e-learning business. **Market Evidence** There are successful e-learning businesses that rely on a subscription model for monetization, by giving users an option to subscribe to access the course library by using one or more plans. Unscreen found the subscription model to be effective for businesses and websites that focus on eLearning after analyzing data of their customers and users (source). Here’s a list of some services support a subscription option or it’s the primary element of their business model: - LinkedIN Learning - Thinkific - Udemy - Coursera Plus Additionally, multiple customers and prospective customers have requested Edly’s