PayPal now supports pay-by-QRCode, so I added it.

[Liam0205]

PR Checklist

• The commit message follows guidelines for NexT.
• Tests for the changes was maked (for bug fixes / features).
  • Muse | Mist have been tested.
  • Pisces | Gemini have been tested.
• Docs in NexT website have been added / updated (for features).

PR Type

• Bugfix.
• Feature.
• Code style update (formatting, local variables).
• Refactoring (no functional changes, no api changes).
• Build related changes.
• CI related changes.
• Documentation content changes.
• Other... Please describe:

What is the current behavior?

Issue resolved: #717

What is the new behavior?

• Screenshots with this changes: N/A
• Link to demo site with this changes: N/A

How to use?

In NexT _config.yml:

reward:
  #wechatpay: /images/wechatpay.png
  #alipay: /images/alipay.png
  paypal: /images/paypal.png
  #bitcoin: /images/bitcoin.png

Does this PR introduce a breaking change?

• Yes.
• No.