Keep viewBox attribute on optimized SVGs

[fturmel]

Closes #1759

This was introduced with #1745, svgo defaults strip-out the viewBox attribute, which is functional and not a safe optimization to blindly apply.

For reference: https://developer.mozilla.org/en-US/docs/Web/SVG/Attribute/viewBox

When an SVG contains a viewBox attribute (often in combination with a preserveAspectRatio attribute), a transform stretches or resizes the SVG viewport to fit a particular container element.

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[coderpro1234-2]

Preview fixes issue #1759

[shiffman]

Thank you @fturmel apologies it took me so long to review this. The fix looks great to me, really appreciate you taking the time!