more old-browser support:

* polyfill Set() for gridview (ie9, ie10)
* navpane: do full-page nav if history api is ng (ie9)
* show markdown as plaintext if rendering fails (ie*)
* text-editor: hide preview pane if it doesn't work (ie*)
* explicitly hide toasts on close (ie9, ff10)

copyparty/web/browser.css

@@ -818,6 +818,10 @@ html.y #path a:hover {
 .logue:empty {
 	display: none;
 }
+.logue.raw {
+	white-space: pre;
+	font-family: 'scp', 'consolas', monospace;
+}
 #doc>iframe,
 .logue>iframe {
 	background: var(--bgg);