Add a view component for rendering thumbnails for media assets. This lets us properly show thumbnails on the upload listing page and the media assets listing page, including support for high pixel density thumbnails and video length icons for videos. Fixes not being able to see thumbnails on the /media_assets page. This is mostly copy/pasted from the post preview component. FIXME: don't duplicate code.
Components
Components are used to encapsulate common UI widgets used throughout the site. This includes things like comments, tag lists, post thumbnails, paginators, and other things used repeatedly throughout the site. Components encapsulate a widget's HTML, CSS, and Javascript together so that all the code for a component is in a single place.
This uses the ViewComponent library.
Components are similar in concept to React components.