Merge commit from fork

* Rework frontend code interacting directly with DOM

Rework frontend code that uses user-supplied data to render things like
comments and notes. In places where using React's built-in sanitization
is possible, the feature is used. In other places, where some markup
might be necessary, DOMPurify is used to sanitize the HTML before
rendering it.

Solves: GHSA-rh3r-8pxm-hg4w

* Remove test post DOM rework

* fixup! Rework frontend code interacting directly with DOM

ui/src/common/SafeHTML.jsx

@@ -0,0 +1,29 @@
+import DOMPurify from 'dompurify'
+import { Fragment, useMemo } from 'react'
+
+export const SafeHTML = ({
+  children,
+}) => {
+  const purified = useMemo(() => {
+    const purify = DOMPurify()
+
+    purify.addHook('afterSanitizeElements', async (node) => {
+      if (node instanceof HTMLElement) {
+        // Set referrer-policy for elements with src
+        switch (node.tagName.toLowerCase()) {
+          case 'a':
+          case 'area':
+          case 'img':
+          case 'video':
+          case 'iframe':
+          case 'script':
+            node.setAttribute('referrer-policy', 'no-referrer')
+        }
+      }
+    })
+
+    return purify.sanitize(children, { ADD_ATTR: ['referrer-policy'] })
+  }, [children])
+
+  return <Fragment dangerouslySetInnerHTML={{ __html: purified }} />
+}