Fix various minor code discussions bugs, make tweaks
Created by: sqs
Fixes bugs and makes minor improvements to code discussions. This addresses issues I have experienced while using and demoing code discussions.
(Code discussions is still an experimental feature. This PR doesn't mean we are prioritizing it for a general-availability release or anything. I just was having fun investigating these bugs.)
Related: https://github.com/sourcegraph/sourcegraph-code-discussions/pull/13