[BUG] Fix manual merge button

- Backport of #2696
- Add `form-fetch-action` to indicate that this form POST to an link
that returns JSON and thus should be handled by Javascript code.
- Found by @fnetx
- Regression of https://codeberg.org/forgejo/forgejo/pulls/1793

(cherry picked from commit 9551c1a6f8)

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 action="{{.Link}}/merge" method="post" class="form-fetch-action">
 						{{.CsrfTokenHtml}}
 						<div class="field">
 							<input type="text" name="merge_commit_id" placeholder="{{ctx.Locale.Tr "repo.pulls.merge_commit_id"}}">