Bugfixes, features, BTCPay 2.0 compat

[ndeet]

Fixes #55,#54,#53,#52

• Feature: Add option to notify customers on refund order notes.
• Feature: BTCPay Server 2.0.0 compatibility.
• Fix: Make sure to not process orders if the assigned payment gateway is not one of BTCPay.
• Fix: Make sure payment methods are set on refunds.
• Fix: Wrong currency in refund comment.
• Fix: Deprecation warnings.
• Maintenance: Update NodeJS dependencies.
• Maintenance: Update PHP library to v2.7.0.