Turns out we were accidentally using the destination knot as the
sourceUrl, and git was understandably annoyed. This also improves the
error checking when fetching the existingRepo from the database to
verify the forkName won't clash.

Forces smaller viewports to w-full all comments; on bigger screens we set a max-width of 3/5.

Thanks @dvjn.dev!

Also sets up a pages/markup package with extensible rendering for future markup languages that we may support.
We may also want an interface or something for markup.RenderMarkdown (maybe just markup.Render), but don't have to bother with that now.