ui: Use web components to render server-side HTML

ui/src/renderedHtml.js

@@ -0,0 +1,24 @@
+// This element allows Inbucket to draw server rendered HTML, aka HTML email.
+// https://leveljournal.com/server-rendered-html-in-elm
+
+customElements.define(
+  "rendered-html",
+  class RenderedHtml extends HTMLElement {
+    constructor() {
+      super()
+      this._content = ""
+    }
+
+    set content(value) {
+      if (this._content === value) {
+        return
+      }
+      this._content = value
+      this.innerHTML = value
+    }
+
+    get content() {
+      return this._content
+    }
+  }
+)