Also make the repo action button font size text-sm.

drive-by fixes:
- make the edit icon for repo-descriptions more compact
- do not double-pad rendered blob contents

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!