Remove keywords in Theme-Next

[xu-song]

PR Checklist

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• The commit message follows our guidelines.
• Tests for the changes have been added (for bug fixes / features).
  • Muse | Mist have been tested.
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PR Type

What kind of change does this PR introduce?

• 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?

There is keywords configuration both in Hexo and Theme-Next.

• Hexo generates keywords from page.keywords, page.tags and config.keywords. source code
• Next generates keywords from page.keywords, page.tags and theme.keywords. source code

What is the new behavior?

Remove keywords configuration in Theme-Next.
It works fine with one configuration.

Does this PR introduce a breaking change?

• Yes.
• No.

[ivan-nginx]

Addition to #115 pull.

---

So, after this pull there are breaking changes — need to move keywords from NexT config to Hexo config:

hexo_dir/themes/next/_config.yml

-# Set default keywords (Use a comma to separate)
-keywords: "Hexo, NexT"

hexo_dir/_config.yml

+# Set default keywords (Use a comma to separate)
+keywords: "Hexo, NexT"

And this keywords will be for homepage only.

For other posts there are no breaking changes and tags/keywords can be setted in old style:

...
tags: [Sidebar, TOC, test]
...

OR

...
tags:
  - Sidebar
  - TOC
  - test
...