Add polyfill for browsers not supporting srcdoc

plugins/base/public/assets/script.js

@@ -1,8 +1,18 @@
 var emailFrame = document.getElementById("email-frame");
 if (emailFrame) {
+	// Resize the frame with its content
 	var resizeFrame = function() {
 		emailFrame.style.height = emailFrame.contentWindow.document.documentElement.scrollHeight + "px";
 	};
 	emailFrame.addEventListener("load", resizeFrame);
 	emailFrame.contentWindow.addEventListener("resize", resizeFrame);
+
+	// Polyfill in case the srcdoc attribute isn't supported
+	if (!emailFrame.srcdoc) {
+		var srcdoc = emailFrame.getAttribute("srcdoc");
+		var doc = emailFrame.contentWindow.document;
+		doc.open();
+		doc.write(srcdoc);
+		doc.close();
+	}
 }

plugins/base/public/message.html

@@ -113,7 +113,6 @@
 {{if .Body}}
 	<p><a href="{{.Message.Uid}}/reply?part={{.PartPath}}">Reply</a></p>
 	{{if .IsHTML}}
-		<!-- TODO: add a src fallback -->
 		<!-- allow-same-origin is required to resize the frame with its content -->
 		<!-- allow-popups is required for target="_blank" links -->
 		<iframe id="email-frame" srcdoc="{{.Body}}" sandbox="allow-same-origin allow-popups"></iframe>