* Increase the default thumbnail size from small (150x150) to medium (180x180).
* Change the mobile layout to use three posts per row instead of two for small thumbnails.
Parent/child posts are still 150x150 to avoid taking up even more space above posts.
Show the length of videos and animated posts in the thumbnail. The
length is shown the top left corner in MM:SS format. This replaces the
play button icon.
Show a speaker icon instead of a music note icon for posts with sound.
Doing this requires doing `.includes(:media_asset)` in a bunch of
places to avoid N+1 queries when we access the post's duration.
Refactor the post preview html to use the ViewComponent framework. This
lets us encapsulate all the HTML, CSS, and helper methods for a UI
component in a single place.
See https://viewcomponent.org.
