Make sidebar position optional for all schemes

[stevenjoezhang]

Breaking Change

sidebar:
-  # Sidebar Position, available values: left | right (only for Pisces | Gemini).
+  # Sidebar Position, available values: left | right.
  # Sidebar Position.
  position: left
  #position: right

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines.
• 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

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?

Issue resolved: #164

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:

Does this PR introduce a breaking change?

• Yes.
• No.

https://github.com/theme-next/hexo-theme-next/pull/156/files#diff-90c3d2a8522cb0e6c0ac99af79e65607R9

[stevenjoezhang]

Need to test if the bug in #164 still exists

[stevenjoezhang]

I suggest merging this PR first. For the issues it may cause, solve them in #951

[jiangtj]

O.O

[stevenjoezhang]

@jiangtj already fixed in #951