Wagtail SVG icons are now generally available

[joshuadavidthomas]

Look at that, the two Wagtail repo links I referenced in the README are closed, and there is a public documentation page on adding SVG icons (ref wagtail/wagtail#6028).

Need to update the README to remove the disclosure. Should also probably bump to v1 so I can promptly bump to v2 for the new heroicons version.