feat: add copyright setting for post

[fish-404]

PR Checklist

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

PR Type

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

What is the current behavior?

Issue resolved: N/A

According to Front Matter | NexT document, there is a setting called copyright, but when I set it to false for one post, the copyright still shown.

What is the new behavior?

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

How to use?

In post front matter, set copyright false will not display copyright notices below the post with theme.creative_commons.license and theme.creative_commons.post enabled.

In post front matter:

---
title: Title
...
copyright: false
---

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