PCI: Implement WordPress.com compatibility

Is your feature request related to a problem?

Our PCI functionality is currently not compatible with WordPress.com, due to the way our WP Rest API is structured.

To quote @travisw:

It appears the way certain API related code is currently implemented in the plugin, might be causing issues when running on WordPress.com. Specifically, registering the top-level namespace wp-parsely/v1 isn’t allowed like it might be on standalone WordPress installs.

We might need to think about modifying the plugin code, to allow the API namespace to be varied depending on if it’s running on WordPress.com or not. It’d need to be done both on the back-end (when registering endpoints) and on the front-end (when making API requests).

Describe the solution you'd like

@travisw has also provided some proof of concept code to us, which verifies that this solution should work. As such, we just need to see how we would implement this in the plugin, both on the back-end and front-end.

Start by locating where the plugin registers the wp-parsely/v1 REST API namespace and where the front end constructs requests to it. Review the referenced proof-of-concept code if available and trace both registration and request paths; done means the PCI functionality works with the varied namespace on WordPress.com without breaking standalone WordPress.