* 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
