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](https://github.com/go-gitea/gitea/assets/2114189/c94ba958-aa46-4747-8ddf-6584deeed25c)

</details>

modules/httplib/url.go

@@ -0,0 +1,27 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package httplib
+
+import (
+	"net/url"
+	"strings"
+
+	"code.gitea.io/gitea/modules/setting"
+)
+
+// IsRiskyRedirectURL returns true if the URL is considered risky for redirects
+func IsRiskyRedirectURL(s string) bool {
+	// Unfortunately browsers consider a redirect Location with preceding "//", "\\", "/\" and "\/" as meaning redirect to "http(s)://REST_OF_PATH"
+	// Therefore we should ignore these redirect locations to prevent open redirects
+	if len(s) > 1 && (s[0] == '/' || s[0] == '\\') && (s[1] == '/' || s[1] == '\\') {
+		return true
+	}
+
+	u, err := url.Parse(s)
+	if err != nil || ((u.Scheme != "" || u.Host != "") && !strings.HasPrefix(strings.ToLower(s), strings.ToLower(setting.AppURL))) {
+		return true
+	}
+
+	return false
+}

modules/httplib/url_test.go

@@ -0,0 +1,38 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package httplib
+
+import (
+	"testing"
+
+	"code.gitea.io/gitea/modules/setting"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestIsRiskyRedirectURL(t *testing.T) {
+	setting.AppURL = "http://localhost:3000/"
+	tests := []struct {
+		input string
+		want  bool
+	}{
+		{"", false},
+		{"foo", false},
+		{"/", false},
+		{"/foo?k=%20#abc", false},
+
+		{"//", true},
+		{"\\\\", true},
+		{"/\\", true},
+		{"\\/", true},
+		{"mail:a@b.com", true},
+		{"https://test.com", true},
+		{setting.AppURL + "/foo", false},
+	}
+	for _, tt := range tests {
+		t.Run(tt.input, func(t *testing.T) {
+			assert.Equal(t, tt.want, IsRiskyRedirectURL(tt.input))
+		})
+	}
+}