Add Referrer Policy Support for Image Privacy Protection

[AldenWangExis]

Type of change

• New feature (non-breaking change which adds functionality)

Description

This PR adds referrer policy support to prevent HTTP Referer headers from being sent when loading images. This enhancement is particularly useful for:

1. Privacy protection: Prevents external image hosts from tracking where images are being displayed
2. Hotlink protection bypass: Enables displaying images from platforms with referrer-based hotlink protection (e.g., WeChat Official Account images)
3. Enhanced security: Reduces information leakage to third-party image servers

Features #2586

Changes Made

1. Meta Tag in Head (head.html)
2. Image Tag Attributes (refactor-content.html)
3. Sidebar Avatar (sidebar.html)

Technical Details

• Backward Compatible: These changes do not break any existing functionality
• Standards Compliant: Uses standard HTML5 referrerpolicy attribute
• Comprehensive Coverage: Applies to all images rendered through Jekyll/Liquid templates
• Performance Impact: Minimal (only adds one HTML attribute per image)

Testing

Tested with:

• WeChat Official Account images (mmbiz.qpic.cn domain)
• Various image formats (jpg, png, gif)
• Both lazy-loaded and LQIP images
• Avatar images in sidebar
• Images with different Kramdown attributes (.shadow)