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>

5 files changed, 89 insertions, 9 deletions

diff --git a/modules/context/base.go b/modules/context/base.go
index 5ae5e65d3e..c8238050f9 100644
--- a/modules/context/base.go
+++ b/modules/context/base.go
@@ -136,6 +136,10 @@ func (b *Base) JSONRedirect(redirect string) {
 	b.JSON(http.StatusOK, map[string]any{"redirect": redirect})
 }
 
+func (b *Base) JSONError(msg string) {
+	b.JSON(http.StatusBadRequest, map[string]any{"errorMessage": msg})
+}
+
 // RemoteAddr returns the client machine ip address
 func (b *Base) RemoteAddr() string {
 	return b.Req.RemoteAddr
diff --git a/modules/context/context_response.go b/modules/context/context_response.go
index 1f215eb8ad..88e375986c 100644
--- a/modules/context/context_response.go
+++ b/modules/context/context_response.go
@@ -16,6 +16,7 @@ import (
 
 	user_model "code.gitea.io/gitea/models/user"
 	"code.gitea.io/gitea/modules/base"
+	"code.gitea.io/gitea/modules/httplib"
 	"code.gitea.io/gitea/modules/log"
 	"code.gitea.io/gitea/modules/setting"
 	"code.gitea.io/gitea/modules/templates"
@@ -49,14 +50,7 @@ func (ctx *Context) RedirectToFirst(location ...string) {
 			continue
 		}
 
-		// 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(loc) > 1 && (loc[0] == '/' || loc[0] == '\\') && (loc[1] == '/' || loc[1] == '\\') {
-			continue
-		}
-
-		u, err := url.Parse(loc)
-		if err != nil || ((u.Scheme != "" || u.Host != "") && !strings.HasPrefix(strings.ToLower(loc), strings.ToLower(setting.AppURL))) {
+		if httplib.IsRiskyRedirectURL(loc) {
 			continue
 		}
 
diff --git a/modules/httplib/url.go b/modules/httplib/url.go
new file mode 100644
index 0000000000..14b95898f5
--- /dev/null
+++ b/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
+}
diff --git a/modules/httplib/url_test.go b/modules/httplib/url_test.go
new file mode 100644
index 0000000000..72033b1208
--- /dev/null
+++ b/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))
+		})
+	}
+}
diff --git a/modules/test/utils.go b/modules/test/utils.go
index 282895eaa9..2917741c45 100644
--- a/modules/test/utils.go
+++ b/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 {