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/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

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
 		}