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/internal/httputil/httputil.go

@@ -0,0 +1,42 @@
+package httputil
+
+import (
+	"encoding/json"
+	"io"
+	"net/http"
+
+	"github.com/seaweedfs/seaweedfs/weed/glog"
+)
+
+const MaxJSONBodyBytes = 1 << 20
+
+func NewJSONMaxReader(w http.ResponseWriter, r *http.Request) io.Reader {
+	return http.MaxBytesReader(w, r.Body, MaxJSONBodyBytes)
+}
+
+func DecodeJSONBody(r io.Reader, v interface{}) error {
+	decoder := json.NewDecoder(r)
+	return decoder.Decode(v)
+}
+
+func WriteJSON(w http.ResponseWriter, status int, payload interface{}) {
+	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(status)
+	if payload == nil {
+		return
+	}
+	if err := json.NewEncoder(w).Encode(payload); err != nil {
+		glog.Errorf("failed to encode JSON response (status=%d, payload=%T): %v", status, payload, err)
+	}
+}
+
+func WriteJSONError(w http.ResponseWriter, status int, message string) {
+	WriteJSON(w, status, map[string]string{"error": message})
+}
+
+func DefaultQuery(value, fallback string) string {
+	if value == "" {
+		return fallback
+	}
+	return value
+}