Don't try to redirect to the feed preview page

Since 1bcf39f the feed preview page
isn't web accessible anymore, so we also can't redirect to it.
This would only break feed previews for feeds that use an actual
RSS/Atom mime-type.

Fixes #477

background/background.js

@@ -453,9 +453,8 @@ chrome.webRequest.onHeadersReceived.addListener(details => {
   // Atom or RSS MIME type, redirect to preview page
   const type = header.value.toLowerCase().replace(/^\s+|\s*(?:;.*)?$/g, "");
   if (type == "application/rss+xml" || type == "application/atom+xml") {
-    return {
-      redirectUrl: getSubscribeURL(details.url),
-    };
+    FeedPreview.show(details.tabId, details.url);
+    return {cancel: true};
   }
 
   // XML MIME type, try sniffing for feed content