Use fetch to send requests to create issues/comments (#25258)

Follow #23290

Network error won't make content lost. And this is a much better
approach than "loading-button".

The UI is not perfect and there are still some TODOs, they can be done
in following PRs, not a must in this PR's scope.

<details>


![image](c94ba958-aa46-4747-8ddf-6584deeed25c)

</details>

modules/test/utils.go

@@ -5,12 +5,29 @@ package test
 
 import (
 	"net/http"
+	"net/http/httptest"
 	"strings"
+
+	"code.gitea.io/gitea/modules/json"
 )
 
 // RedirectURL returns the redirect URL of a http response.
+// It also works for JSONRedirect: `{"redirect": "..."}`
 func RedirectURL(resp http.ResponseWriter) string {
-	return resp.Header().Get("Location")
+	loc := resp.Header().Get("Location")
+	if loc != "" {
+		return loc
+	}
+	if r, ok := resp.(*httptest.ResponseRecorder); ok {
+		m := map[string]any{}
+		err := json.Unmarshal(r.Body.Bytes(), &m)
+		if err == nil {
+			if loc, ok := m["redirect"].(string); ok {
+				return loc
+			}
+		}
+	}
+	return ""
 }
 
 func IsNormalPageCompleted(s string) bool {