PayPal REST

[dparker1005]

This work is not ready for production environments.

[dparker1005]

At this point, the gateway feels relatively stable but is still not ready for a full release. This PR hides the gateway behind a constant that needs to be set with the following code:
define( 'PMPRO_PAYPALREST_BETA', true );

There is a known incompatibility with the PMPro Pay By Check Add On (credit card fields showing for PayPal REST) that will need to be resolved in PBC or by core handling multiple gateways natively. Similar incompatibilities exist with the Add PayPal Express Add On, though that Add On doesn't really make sense to use alongside PayPal REST.

There is also an open question around how REST API credentials should be disabled. Right now, the "disconnect" option just deletes the keys from the database but they could still be used elsewhere.

On our oauth server, we also still need to add PayPal Multiparty keys for the Stranger Studios platform account.

[dparker1005]

Also important to note:

• Webhooks can only be set up using https://
• Sites labeled as “high risk” by this site won’t be able to receive webhooks: https://urlfiltering.paloaltonetworks.com (PayPal mentions this in their “Troubleshooting delivery problems” section here: https://developer.paypal.com/api/rest/webhooks/rest/)