keyboard shortcut for "show more"/"show all sensitive content"/content-warning expansion

**Is your feature request related to a problem? Please describe.**
I move through posts using keyboard shortcuts, and often come across posts that have a content warning/summary where the details are hidden behind a "Show more" link. But I can't figure out how to expand/show more/unblur the contents while using the keyboard.

**Describe the solution you'd like**
A separate keyboard shortcut to expand show more/show sensitive content links, or re-use of the 'x'/show-thread command to also show more/sensitive content.

**Describe alternatives you've considered**
I can use tab and space within a post to highlight and then activate the show more button, but I find that more confusing and harder to remember. And if you hit the spacebar when it's not highlighted, the typical behavior will scroll down away from the content that you're trying to expand.

Start by locating the keyboard-shortcut handling and the post controls for “Show more” and sensitive-content expansion. Trace how the existing show-thread shortcut works, then determine whether the new behavior should use a separate shortcut or reuse “x”; done means keyboard users can expand or unblur the content without tabbing to the button, while existing scrolling and shortcuts continue to work.

Written by the indexing model from the issue text.