feat: add img prefix for i.redd.it

"i.redd.it/<imgfile>" redirects "libredd.it/<imgfile>".  Make it
redirect to "libredd.it/img/<imgfile>".  Tested only for "libredd.it".

src/pages/background/background.js

@@ -448,6 +448,9 @@ function redirectReddit(url, initiator, type) {
   if (type !== "main_frame" || url.pathname.match(redditBypassPaths)) {
     return null;
   }
+  if (url.host === "i.redd.it") {
+    return `${redditInstance}/img${url.pathname}${url.search}`;
+  }
   return `${redditInstance}${url.pathname}${url.search}`;
 }