Admin UI: replace gin with mux (#8420)

* Replace admin gin router with mux

* Update layout_templ.go

* Harden admin handlers

* Add login CSRF handling

* Fix filer copy naming conflict

* address comments

* address comments

weed/admin/dash/http_helpers.go

@@ -0,0 +1,24 @@
+package dash
+
+import (
+	"io"
+	"net/http"
+
+	"github.com/seaweedfs/seaweedfs/weed/admin/internal/httputil"
+)
+
+func writeJSON(w http.ResponseWriter, status int, payload interface{}) {
+	httputil.WriteJSON(w, status, payload)
+}
+
+func writeJSONError(w http.ResponseWriter, status int, message string) {
+	httputil.WriteJSONError(w, status, message)
+}
+
+func decodeJSONBody(r io.Reader, v interface{}) error {
+	return httputil.DecodeJSONBody(r, v)
+}
+
+func newJSONMaxReader(w http.ResponseWriter, r *http.Request) io.Reader {
+	return httputil.NewJSONMaxReader(w, r)
+}