enhancement: add signoff option in commit form (#14516)

Signed-off-by: a1012112796 <1012112796@qq.com>

templates/repo/editor/commit_form.tmpl

@@ -14,6 +14,12 @@
 		<div class="field">
 			<textarea name="commit_message" placeholder="{{.i18n.Tr "repo.editor.commit_message_desc"}}" rows="5">{{.commit_message}}</textarea>
 		</div>
+		<div class="inline field">
+			<div class="ui checkbox">
+				<input name="signoff" type="checkbox" tabindex="0" class="hidden">
+				<label>{{.i18n.Tr "repo.editor.signoff_desc"}}</label>
+			</div>
+		</div>
 		<div class="quick-pull-choice js-quick-pull-choice">
 			<div class="field">
 				<div class="ui radio checkbox {{if not .CanCommitToBranch.CanCommitToBranch}}disabled{{end}}">