Move media description input to a modal and unite that modal with
the focal point modal. Add a hint about choosing focal points, as
well as a preview of a 16:9 thumbnail. Enable the user to watch
the video next to the media description input.
Fix#8320Fix#6713
* Add focus param to media API, center thumbnails on focus point
* Add UI for setting a focal point
* Improve focal point icon on upload item
* Use focal point in upload preview
* Add focalPoint property to ActivityPub
* Don't show focal point button for non-image attachments
