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 #8320
Fix #6713

* Do not push DMs into the home feed

* Show DMs column after sending a DM, if DMs column is not already shown

* 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

* Fix #117 - Add ability to specify alternative text for media attachments

- POST /api/v1/media accepts `description` straight away
- PUT /api/v1/media/:id to update `description` (only for unattached ones)
- Serialized as `name` of Document object in ActivityPub
- Uploads form adjusted for better performance and description input

* Add tests

* Change undo button blend mode to difference