Calculate --sidebar-wrapper-height in pure CSS

[stevenjoezhang]

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).
• Documentation.
• Translation.
• Other... Please describe:

What is the current behavior?

Issue resolved:

See also #243

What is the new behavior?

• Link to demo site with this changes:
• Screenshots with this changes:

Refactor sidebar-inner with flex layout. Completely remove the initSidebarDimension function and --sidebar-wrapper-height

How to use?

In NexT _config.yml:

[coveralls]

Coverage remained the same at 94.468% when pulling 28fa98e on sidebar into c377515 on master.

[stevenjoezhang]

@tusooa Can you help review this pr? The complex calculations of --sidebar-wrapper-height can be implemented with a more robust CSS flex layout