Fix the /pools/gallery page layout being broken by 8841de68a.
This required refactoring the PostGalleryComponent to take a set of
PostPreviewComponents instead of a set of Posts.
The upshot is that it's technically possible to have adjustable
thumbnail sizes on the pool gallery page now (although this is not yet
exposed in the UI).
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.
