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.
