Fix: Generate/regenerate/rescan buttons do not work in media modal when editing a post

[dinhtungdu]

Description of the Change

The buttons in the media only work under the Media library page. This happens because we only listen to edit:attachment event triggered when you open the media modal of an image on the Media library page. This PR adds a listener to another event to make the buttons work when editing a post.

Verification Process

1. Build new assets.
2. Edit a post using the block editor, see buttons work.
3. Edit a post using the classic editor, see buttons work.
4. Edit an attachment, see buttons work.

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests passed.

Applicable Issues

Close #292.

Changelog Entry

Fixed: Ensure Image Processing buttons in the media modal work when editing posts.

[helen]

This does work now for existing items; however, it doesn't seem to work for new uploads within that modal "session" - they'll work if you close the modal and reopen it, though. Maybe it has to be bound a different way for that? Perhaps not a blocker since there's a workaround but wanted to note it in case we want to log it as as separate issue and release with it this way, which is already better.

[dinhtungdu]

they'll work if you close the modal and reopen it

@helen they also work if you unselect then select again. I tried to fix this issue as well but it's not simple as it seems. I will look at it again next week.

[helen]

Just noting in writing that while less than great the fix is still better than before so we can figure out the final piece (not having to click away and then back to have the buttons work) later on.