Add handling of comments with giscuss

[suchja]

The original theme has no visual support for comments. This needs to be added. Additionally the same integration of giscuss as done in CleanBlog theme needs to be supported here. Not sure whether this includes any changes. Probably everything related to Giscuss comes already with Statiq!

The CleanBlog theme has some partial Razor pages for the comments. With those the user of the theme can modify the appearance/behavior? of the comments. This should be done here as well.