Installer: Add Cache-Control header to static assets

This fixes UX where Launcher iframe blanks when navigating in the app

Change-Id: I2966ef383c77d2986b6892f3828581b7785e90b0
diff --git a/core/installer/welcome/launcher.go b/core/installer/welcome/launcher.go
index 1d51ee8..288fc60 100644
--- a/core/installer/welcome/launcher.go
+++ b/core/installer/welcome/launcher.go
@@ -134,7 +134,7 @@
 }
 
 func (s *LauncherServer) Start() {
-	http.Handle("/static/", http.FileServer(http.FS(files)))
+	http.Handle("/static/", cachingHandler{http.FileServer(http.FS(files))})
 	http.HandleFunc("/", s.homeHandler)
 	log.Fatal(http.ListenAndServe(fmt.Sprintf(":%d", s.port), nil))
 }