fix(giscus): add missing attributes and allow local theme

[ReenigneArcher]

This PR adds some missing giscus attributes. I assume these were not available when giscus was added originally.

Additionally, there is a logic change that allows a local css file for the giscus theme. I tried to test this locally and it requires CORS to be disabled, I am not sure how this would work on a GitHub hosted site, but in any event it allows to specify a theme like /assets/css/giscus.css or any local path that starts with /. Normally giscus will try to get any non full url theme from their own server/api.

https://giscus.app/

Here is a screenshot of evidence of the a custom theme css working for giscus.

[ahmadwardat]

Yes

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