Add a hidden option for 540px size thumbnails. This option currently
isn't listed in the thumbnail size menu, but it can be enabled with an
URL param: https://danbooru.donmai.us/posts?size=540.
* 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.
* Bottom-align thumbnails on desktop. This is so that vote buttons line
up beneath thumbnails. Also so that pool titles line up on the pool
gallery page. This is desktop only because it tends to leave large
gaps above thumbnails on mobile.
* Make thumbnails fixed-height so that the layout doesn't shift as
thumbnails are loaded.
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.
