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).
Factor out thumbnail galleries into a PostGallery component.
This changes the html structure so that post galleries on all pages are
always wrapped in a `.posts-container` class. This fixes an issue with
thumbnails on the pool show page not being aligned correctly on mobile,
like they are on the post index page. This also affected thumbnail
galleries on other pages, like wiki pages and user profiles.
Add a curated posts page at /explore/posts/curated. Curated posts are
the most favorited posts by contributor-level users (users with
unlimited upload permissions).
Also add an order:curated tag using for use in regular searches.
