Make posts's status clearer on "Add more to report" Moderation UI page

[gunchleoc]

Pitch

In the moderation UI, when "Add more to report" is selected, do not list already selected posts, or give them a checkmark.

Not showing them at all would unclutter the scree, since there's a separate "Remove" function.

Motivation

In the moderation UI, when "Add more to report" is selected, already selected posts appear, and they appear unchecked. This makes it unclear which posts had already been selected previously.

[ThisIsMissEm]

This would be a really good idea! I've already made some small changes there for #30813

Specifically contextually changing "Report" to "Add to Report #123" for the button in the table, if you're doing something from a specific report.