Improves the subscription flow for anonymous users by making the routes available, and showing a login button. Clicking login from this page will save a `destination_url` cookie so that when logging in they're redirected back to the subscription page they were at.
The code in the plugin needed a dramatic cleanup. This refactor collapses the Plan/Product/Subscription controllers on the backend into one new controller: `SubscribeController`.
This reduces N+1 calls to the back end during the subscription process and simplifies use of the code.
I've also removed a bunch of dead code and refactored some logic into methods for easier readability. No feature/functionality changes in this commit; only refactoring. However, refactoring will allow for implementation of better anonymous user handling, so this is largely a foundation to enable making that change.
Justin DiRose