We were inheriting the default style for the CodeMirror cursor in some

uses, which resulted in a black-on-black cursor. We were overriding this

correctly in rich text fields, but not elsewhere.
This change moves the fixed styling into the common CodeMirror

definition, fixing that. While I was at it, I also removed the

now-redundant selection styling, utilizing the common definition that

was placed there before.

Tested the CodeMirror usage in both review request fields and in the

API Token Policy editor, checking both the cursor and the selections to

make sure they were visible in light and dark modes.