* Pass through video description as necessary.

* Add title to video, matching aria-label.

* Prevent default event handling when clicking on the video position slider

This prevents accidental text selection when clicking on the position slider.

* Fix bug when clicking on video position slider before starting the video

* Slightly more aggressive video preloading

- Preload video metadata if the video is loaded in detailed view, as it is
  likely to get played, and metadata is useful for seeking in the video.
- Preload video data if it's fullscreen as it is extremely likely to get
  played right after being put in fullscreen (although those are two steps).
- Preload video data if the user has clicked the position slider, as the video
  will play as soon as the mouse button is released, and video metadata is
  needed to properly seek into the video.

* Responsively enforce 16:9 ratio on all media thumbnails in web UI

Also change video player behaviour to "contain" rather than
"cover" videos that don't fit the ratio, unlike images and GIFs,
it's expected that a video is shown fully.

* Fix spacing issues and remove floor

* Remove floor

* 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

* Toggle contain:strict on fullscreen

* Fix scss lint issue

* fix scss whitespace lint issue

* 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

* Redesign video player

* Use new video player on static public pages too

* Use media gallery component on static public pages too

* Pause video when hiding it

* Full-screen sizing on WebKit

* Add aria labels to video player buttons

* Display link card on public status page

* Fix fullscreen from modal sizing issue

* Remove contain: strict property to fix fullscreen from columns