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.
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).
Disable cropped (square) thumbnails on mobile. Use regular uncropped
thumbnails instead.
This is for a few reasons:
* It made it harder to support multiple thumbnail sizes or file formats,
since we need a cropped and uncropped version for every size and format.
* The cropping algorithm wasn't that great and sometimes cropped out
important parts of the image.
* The thumbnail type (cropped or uncropped) was automatically chosen
clientside based on the user's screen size, which made certain things
like adjustable thumbnail sizes more difficult because we didn't know
which thumbnail type the user actually had.
This may return again as an option in the future, but for now it's disabled.
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.
