Apply correct styling for `<pre>` elements

Beforehand, `<pre>` elements would inherit the [`word-wrap`](https://developer.mozilla.org/en-US/docs/Web/CSS/word-wrap) property of
the `.from-content-api` CSS class, which is set to `break-word`.

We don’t want this behaviour for `<pre>` elements: text inside `<pre>` elements
should not be wrapped, and instead the user can scroll horizontally to view the
full line. This mirrors the behaviour in R2 frontend.