feat(problem): show contests used this problem

web/app/controllers/problem.php

@@ -307,7 +307,25 @@ if (UOJContest::cur()) {
 					<?= UOJLocale::get('problems::memory limit') ?>: <?= $memory_limit ? "$memory_limit MB" : "N/A" ?>
 				</div>
 
-				<hr>
+				<hr />
+
+				<?php if (!UOJContest::cur()) : ?>
+					<?php $contest_problems = UOJProblem::cur()->findInContests(); ?>
+					<?php if (!empty($contest_problems)) : ?>
+						<div class="alert alert-light">
+							<h5 class="alert-heading"><?= UOJLocale::get('problems::the problem was used in the following contest') ?>:</h5>
+							<ul class="mb-0">
+								<?php foreach ($contest_problems as $cp) : ?>
+									<?php if ($cp->userCanView(Auth::user())) : ?>
+										<li><?= $cp->contest->getLink(['class' => 'alert-link text-decoration-underline']) ?></li>
+									<?php endif ?>
+								<?php endforeach ?>
+							</ul>
+						</div>
+
+						<hr />
+					<?php endif ?>
+				<?php endif ?>
 
 				<div class="tab-content">
 					<div class="tab-pane active" id="statement">

web/app/locale/problems/en.php

@@ -64,4 +64,5 @@ return [
 	'attachments download' => 'Attachments download',
 	'resources' => 'Resources',
 	'cannot submit' => 'This problem doesn\'t configured <code>submission_requirement</code>, so it cannot be submitted temporarily.',
+	'the problem was used in the following contest' => 'The problem was used in the following contest',
 ];

web/app/locale/problems/zh-cn.php

@@ -63,5 +63,6 @@ return [
 	'test data' => '测试数据',
 	'attachments download' => '附件下载',
 	'resources' => '相关资源',
-	'cannot submit' => '当前题目未配置 <code>submission_requirement</code>，暂时无法提交。'
+	'cannot submit' => '当前题目未配置 <code>submission_requirement</code>，暂时无法提交。',
+	'the problem was used in the following contest' => '该题目被以下比赛使用',
 ];