Remove Giscus from search page

[adamscott]

This moves the Giscus insertion logic inside custom.js.

Technically, it could have worked if I would have done this instead:

{% if (not meta or meta.get('allow_comments') != 'False') and godot_show_article_comments and pagename != 'search' %}

... but it's easier to maintain my way, as we can add easily new pages to ban.

[mhilbrunner]

cc @Calinou

[Calinou]

Tested locally, it works as expected.