Integrate blog/discussions into start page

[SReichelt]

Idea: Use the new "Discussions" feature of GitHub as a basis for a blog that is integrated into the Slate front page. That seems like a good way to have lightweight blog posts with discussion threads.

Unfortunately, discussions are not available via the GitHub API yet, but will hopefully be soon: https://stackoverflow.com/questions/65166895/how-to-query-data-from-github-discussions-using-githubs-graphql-api

There should probably be a discussion category "Blog" for this. It would be nice to be able to restrict the ability to post in this category, but that doesn't seem possible. Alternatively, we can only display blog posts by specific users.