Fix review box viewport overflow issue (#23800)

Fix review box viewport overflow issue (#23800)

Fix regression that came likely from
https://github.com/go-gitea/gitea/pull/23271:

Long lines of text currently cause the review box's CodeMirror element
to resize which apparently is not recognized by [popper's resize
detection](https://popper.js.org/docs/v2/modifiers/event-listeners/) and
which causes the element to go partially out of viewport until a reflow
happens:

![wrap](https://user-images.githubusercontent.com/115237/228673260-f8ffe89e-5724-4fb9-b93f-30b146c4c436.gif)

Fix this by setting the element to a static width derived from viewport
width and remove the previously clumsy media queries.