The work on user badges regressed some of the display of the header area

at the top of reviews and in comments. This wasn't helped by the fact

that this is some of the oldest CSS in the codebases, and it's a huge

rats nest. Fixing this meant fixing it in multiple places and then

dealing with overrides.
Instead of making the situation worse, this change introduces a new

rb-c-discussion-header component that can be used in each of these

areas. It's responsible for showing the author information or title, the

timestamp, and an optional avatar and "new updates" icon.
This can also be put in compact mode, which places the timestamp below

the name. Replies enable this and avatars, whereas the review headers do

not.
A Storybook story has been added to help document and maintain this

component.

Tested this in mobile and desktop modes on all review request entry

boxes and in replies (published, existing draft, and new draft).
Tested in light and dark modes.
All Python and JavaScript unit tests pass.