fix(fancybox): add data-fancybox attribute for fancybox3

[Raincal]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines.
• Tests for the changes have been added (for bug fixes / features).
  • Muse | Mist have been tested.
  • Pisces | Gemini have been tested.
• Docs have been added / updated (for bug fixes / 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 Number(s): N/A

What is the new behavior?

Description about this pull, in several words...

• Screens with this changes: N/A
• Link to demo site with this changes: https://raincal.com/2017/08/07/deploy-flarumChina-with-docker/

How to use?

In NexT _config.yml:

...

Does this PR introduce a breaking change?

• Yes.
• No.

[ivan-nginx]

@Raincal can u test this with fancybox2? All working fine?

[Raincal]

It works fine with fancybox2

[chy9002]

I try to add fancybox3 in my website, but there is a wired issue. Button icons aren't visible, is anybody know how to resolve it?