Experiment: modify KSES filter to allow Font Awesome svg html under lower-privileged conditions

[mlwilkerson]

This PR is an alternative to #272.

Where #272 uses server-side dynamic block block render to avoid storing raw HTML in posts, this one adjusts the KSES filter to allow Font Awesome <svg> HTML to be written into post_content.

Advantages

1. Remains continuous with existing functionality: where raw <svg> HTML is stored in posts, and no server-side rendering is required for front end page loads
2. Works for both icon blocks and rich text icons. Whereas, Experiment: use dynamic block server-side rendering #272 only works for icon blocks.
3. It's a smaller overall change than Experiment: use dynamic block server-side rendering #272

Disadvantages

1. It requires changing global KSES filters which impacts all HTML sanitization