Updating Stripe Payment Gateway

[VAlmea]

Description of PR...

The PR aims to migrate Stripe's payment gateway implementation to use the latest version of the PaymentIntents API, which is SCA-ready.

Changes made

• Update the Stripe.Net SDK to the latest version 40.9.0
• Update the StripeProcessor class. We added all the methods to process a PymentIntents based on the Stripe API doc
• The Stripe PublicKey field was added in the Stripe Options view in the Admin panel
• The Stripe.js library was added on the client-side to process payment methods and trigger the 3D secure Auth

Close #395

[WillStrohl]

@VAlmea Your PR has conflicts. Please review your PR to resolve these conflicts and let me know when this is done. Thank you! :)

Also, there appears to be additional updates that are no related to the PR and the Stripe update that should already be there. Please be sure to address those as well, or let me know why those updates are there. Normally, we'll want a separate Issue and PR for each different type of update.