Add basic PWA offline page

- Remove feed_icons cache because it's causing more problems that it solve.
- Add basic offline mode when using the service worker.
- Starting in Chrome 93, offline mode is going to be a requirement to install the PWA.

https://developer.chrome.com/blog/improved-pwa-offline-detection/#enforcement-starting-chrome-93-august-2021

ui/static/static.go

@@ -127,8 +127,7 @@ func GenerateJavascriptBundles() error {
 	}
 
 	var prefixes = map[string]string{
-		"app":            "(function(){'use strict';",
-		"service-worker": "'use strict';",
+		"app": "(function(){'use strict';",
 	}
 
 	var suffixes = map[string]string{