Icon option to sidebar chat button

[1v9]

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 new 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?

the icon of chat button in sidebar is hardcode -> fa-comment

What is the new behavior?

use any icon in fontawesome v4

How to use?

In NexT _config.yml:

# A button to open designated chat widget in sidebar.
# Firstly, you need enable the chat service you want to activate its sidebar button.
chat:
  enable: true
  service: chatra
  #service: tidio
 +icon: comment # icon in Font Awesome4, set false to disable icon
  text: Chat # button text, change it as you wish

Does this PR introduce a breaking change?

• Yes.
• No.