Fix manual merge form and 404 page templates (#30000)

Partially backport #29985, fix some template errors.

* fix the manual-merge form
* fix the 404 page

templates/repo/issue/view_content/pull.tmpl

@@ -360,7 +360,7 @@
 			{{if and $.StillCanManualMerge (not $showGeneralMergeForm)}}
 				<div class="divider"></div>
 				<div class="ui form">
-					<form action="{{.Link}}/merge" method="post">
+					<form class="form-fetch-action" action="{{.Link}}/merge" method="post">
 						{{.CsrfTokenHtml}}
 						<div class="field">
 							<input type="text" name="merge_commit_id" placeholder="{{ctx.Locale.Tr "repo.pulls.merge_commit_id"}}">