Fix conflict between animation of sub-menu icons and pjax

[foxB612]

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:
  When animation and pjax are both turned on, the sub-menu icons will become invisible after a pjax reload.
• Reason:
  Sub-menu items have opacity of 0, as defined in css together with menu-item, which need to be animated to opacity 1 by velocity. However sub-menu is also in the part that pjax refreshes, and pjax script overlooks them, resulting in invisibility after pjax reload.
• A demo (with Pisces): https://foxb612.github.io/sub-menu-bug.gif

What is the new behavior?

Bug fixed.

Does this PR introduce a breaking change?

• Yes.
• No.

[welcome]

Thanks so much for opening your first PR here!

[claassistantio]

All committers have signed the CLA.

[stevenjoezhang]

Thank you for your contribution! We will investigate and confirm this solution.

[welcome]

Congrats on merging your first pull request here! 🎉 How awesome!