Switch to SVG icons with consistent aspect ratio

[AdamGleave]

All icons are now SVG and have aspect ratio 1. Also add that aspect ratio to CSS class to avoid Cumulative Layout Shift (CLS).

@ccstan99 could you make an attack & defense themed version of the video icon using the swords/shields & add them to this PR? I don't have the original iconography for those. We don't have a defense video yet but we will thanks to you, so would be nice to have an icon for it.

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[ccstan99]

I don't have permission to push to this repo. New SVGs (attack-video.svg & defense-video.svg) are in this folder.

[AdamGleave]

Thanks ChengCheng -- added to PR now