Expose an action to regenerate or refresh the thumbnail

(For reasons to tiresome to explain - current guess is it was something to do with migration...)

It can be helpful or necessary to manually trigger a regeneration or reset of a media items designated thumbnail.

The code already provides an API call to do that pretty easily, but there seems to be no UI for it.
Drupal\media_entity\Entity\Media::automaticallySetThumbnail()

*Ideally* all parts of the system would always know how to keep the thumbnail up to date at all times, but issues ike
#2749695: Thumbnail not updated
#2786881: Recalculate thumbnail size during update
show this is not always straightforward.

Though it could be argued it's not *necessary* - it is *useful* . Ideally, the image_style management would always be up to date as well, yet the core image style management UI provides the 'flush' action on image styles as a utility anyway, and sometimes it's handy - especially during site migrations etc.

Exposing the 'Action' as an admin assistant seems easy and harmless. I did a local trial and it works as needed for me. I'll see if I can get a PR into github..