#7877829 Comments: add buttons to empty spam and trash

Proposed Changes

It’s currently possible to bulk edit comments in Calypso, but not to empty spam or trash with a single action (like you can in wp-admin). This was spotted by Matt in a Jetpack walkthrough recently – see pedxs5-7f-p2.

This PR adds an “empty” button to the spam and trash tabs in Calypso comment management.

Testing Instructions

1. Ensure there is an ‘Empty spam’ button on http://calypso.localhost:3000/comments/spam and an ‘Empty trash’ button on http://calypso.localhost:3000/comments/trash when you have comments in Spam or Trash.
2. Try the button and ensure that it empties all comments from the current section successfully, and gives you adequate information while it does so. The comment counts should be accurate after the emptying has happened.

Pre-merge Checklist

• Has the general commit checklist been followed? (PCYsg-hS-p2)
• Have you written new tests for your changes?
• Have you tested the feature in Simple (P9HQHe-k8-p2), Atomic (P9HQHe-jW-p2), and self-hosted Jetpack sites (PCYsg-g6b-p2)?
• Have you checked for TypeScript, React or other console errors?
• Have you used memoizing on expensive computations? More info in Memoizing with create-selector and Using memoizing selectors and Our Approach to Data
• Have we added the “[Status] String Freeze” label as soon as any new strings were ready for translation (p4TIVU-5Jq-p2)?
• For changes affecting Jetpack: Have we added the “[Status] Needs Privacy Updates” label if this pull request changes what data or activity we track or use (p4TIVU-ajp-p2)?