Fix thumbnail CSS to not use `width: auto`, because that made the page
reflow. It made thumbnails start out at 0x0 size, then resize to the
actual size only after the image was loaded.
Three thumbnails per row was pretty tight for most phones since the most
common phone resolution is 360x640. Two thumbnails per row lets us have
thumbnails up to 180x180 in size.
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.
