Modernize merge button (#28140)

- Make use of the `form-fetch-action` for the merge button, which will
automatically prevent the action from happening multiple times and show
a nice loading indicator as user feedback while the merge request is
being processed by the server.
- Adjust the merge PR code to JSON response as this is required for the
`form-fetch-action` functionality.
- Resolves https://codeberg.org/forgejo/forgejo/issues/774
- Likely resolves the cause of
https://codeberg.org/forgejo/forgejo/issues/1688#issuecomment-1313044

(cherry picked from commit 4ec64c19507caefff7ddaad722b1b5792b97cc5a)

Co-authored-by: Gusted <postmaster@gusted.xyz>

tests/integration/pull_merge_test.go

@@ -45,7 +45,14 @@ func testPullMerge(t *testing.T, session *TestSession, user, repo, pullnum strin
 		"_csrf": htmlDoc.GetCSRF(),
 		"do":    string(mergeStyle),
 	})
-	resp = session.MakeRequest(t, req, http.StatusSeeOther)
+	resp = session.MakeRequest(t, req, http.StatusOK)
+
+	respJSON := struct {
+		Redirect string
+	}{}
+	DecodeJSON(t, resp, &respJSON)
+
+	assert.EqualValues(t, fmt.Sprintf("/%s/%s/pulls/%s", user, repo, pullnum), respJSON.Redirect)
 
 	return resp
 }