Related: #23590
Reference:
https://github.com/webcomponents/polyfills/tree/master/packages/webcomponentsjs
It seems that there are some users using old browsers, so the
`window.customElements` need polyfill.
The Custom Elements would help a lot for Gitea's UI problems, including:
* `<span class="js-pretty-number">`
* `<time data-format>`
So it's worth get polyfill.
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Co-authored-by: delvh <dev.lh@web.de>