Fix incorrect redirection when creating a PR fails (#29537)

This is only a quick fix to make it easier to backport.

After this PR gets merged, I will propose a new PR to fix the FIXME.

<details>

![image](https://github.com/go-gitea/gitea/assets/2114189/98d1d5c4-2e79-4a75-80e9-76fd898986e0)

</details>

templates/repo/issue/new.tmpl

@@ -2,14 +2,6 @@
 <div role="main" aria-label="{{.Title}}" class="page-content repository new issue">
 	{{template "repo/header" .}}
 	<div class="ui container">
-		{{if .Flash.WarningMsg}}
-			{{/*
-			There's already an importing of alert.tmpl in new_form.tmpl,
-			but only the negative message will be displayed within forms for some reasons, see semantic.css:10659.
-			To avoid repeated negative messages, the importing here if for .Flash.WarningMsg only.
-			 */}}
-			{{template "base/alert" .}}
-		{{end}}
 		{{template "repo/issue/new_form" .}}
 	</div>
 </div>

templates/repo/issue/new_form.tmpl

@@ -1,10 +1,8 @@
+{{if .Flash}}
+{{template "base/alert" .}}
+{{end}}
 <form class="issue-content ui comment form form-fetch-action" id="new-issue" action="{{.Link}}" method="post">
 	{{.CsrfTokenHtml}}
-	{{if .Flash}}
-		<div class="sixteen wide column">
-			{{template "base/alert" .}}
-		</div>
-	{{end}}
 	<div class="issue-content-left">
 		<div class="ui comments">
 			<div class="comment">