Improve caching, HTTP headers and URL handling (#3594)

* Fix relative URLs
* Improve login base URL and redirects
* Prevent duplicate customlocales requests
* Improve UI base URL handling
* Improve UI embedding
* Improve CSP header
* Add Cache-Control headers to all responses
* Improve CORS responses
* Improve authentication handler
* Add back media timestamp suffixes
* Fix default image handling
* Add default param to other image URLs

internal/manager/downloads.go

@@ -80,6 +80,7 @@ func (s *DownloadStore) Serve(hash string, w http.ResponseWriter, r *http.Reques
 	if f.contentType != "" {
 		w.Header().Add("Content-Type", f.contentType)
 	}
+	w.Header().Set("Cache-Control", "no-store")
 	http.ServeFile(w, r, f.path)
 }