fix(comments/cusdis): use the correct script for the self-hosted version

[hu3rror]

refer: https://cusdis.com/doc#/advanced/sdk

If you are using self-hosted Cusdis, remember changing the data-host and the host in <script> to your own domain

[CaiJimmy]

I think we can refactor the code this way to avoid multiple if sentence.

{{- $host := default "https://cusdis.com" .Site.Params.comments.cusdis.host -}}
<div id="cusdis_thread"
    data-host="{{ $host }}"
    data-app-id="{{ .Site.Params.comments.cusdis.id }}"
    data-page-id="{{ .File.UniqueID }}"
    data-page-url="{{ .Permalink }}"
    data-page-title="{{ .Title }}"></div>
<script async defer src="{{ $host }}/js/cusdis.es.js"></script>

[hu3rror]

Thank you for your code :)