Update SVG sanitizer to conform to SVG 1.1

Original SVG sanitizer would strip out necessary elements and attributes.

Use a custom Loofah scrubber since sanitize 2.x transformers are inadequate to handle case-sensitive SVG attributes since they parse documents as HTML instead of XML, which causes all SVG attribute names (e.g. `viewBox`) to be downcased.

* SVG element list: https://www.w3.org/TR/SVG/eltindex.html
* SVG attribute list: https://www.w3.org/TR/SVG/attindex.html

Closes #14555

See merge request !3401