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>

routers/common/redirect.go

@@ -0,0 +1,26 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package common
+
+import (
+	"net/http"
+
+	"code.gitea.io/gitea/modules/httplib"
+)
+
+// FetchRedirectDelegate helps the "fetch" requests to redirect to the correct location
+func FetchRedirectDelegate(resp http.ResponseWriter, req *http.Request) {
+	// When use "fetch" to post requests and the response is a redirect, browser's "location.href = uri" has limitations.
+	// 1. change "location" from old "/foo" to new "/foo#hash", the browser will not reload the page.
+	// 2. when use "window.reload()", the hash is not respected, the newly loaded page won't scroll to the hash target.
+	// The typical page is "issue comment" page. The backend responds "/owner/repo/issues/1#comment-2",
+	// then frontend needs this delegate to redirect to the new location with hash correctly.
+	redirect := req.PostFormValue("redirect")
+	if httplib.IsRiskyRedirectURL(redirect) {
+		resp.WriteHeader(http.StatusBadRequest)
+		return
+	}
+	resp.Header().Add("Location", redirect)
+	resp.WriteHeader(http.StatusSeeOther)
+}