feat(web): require login only for candidates and system

reaction_board/web/static/index.html

@@ -20,6 +20,7 @@
           <small id="topbar-title">最新动态</small>
         </span>
       </a>
+      <a class="topbar-hint" id="topbar-auth" href="/login" hidden>登录</a>
       <span class="topbar-hint" id="topbar-hint" hidden>预览</span>
     </header>
     <div class="preview-banner" id="preview-banner" hidden>预览数据 · 不影响真实数据库</div>