storage manager: remove hierarchical option.

Remove the `hierarchical` file storage option. This means that image
files are always stored in MD5-based subdirectories, like this:

   https://danbooru.donmai.us/data/original/f3/a7/f3a70a89c350b5ed4db22dbb25b934bb.jpg
   https://danbooru.donmai.us/data/sample/f3/a7/sample-f3a70a89c350b5ed4db22dbb25b934bb.jpg
   https://danbooru.donmai.us/data/preview/f3/a7/f3a70a89c350b5ed4db22dbb25b934bb.jpg

instead of in a single flat directory, like this:

   https://danbooru.donmai.us/data/original/f3a70a89c350b5ed4db22dbb25b934bb.jpg

This option is removed because storing files in a single directory is a
bad idea for large installations, and migrating from a single directory
to subdirectories later is a pain.

Downstream boorus who still have files in the old layout can migrate by
running this script:

   `./script/fixes/077_symlink_subdirectories.rb`

This will create symlinks that redirect the 00-ff subdirectories back to
the current directory, so that you can still store files in a single
directory, but use URLs containing subdirectories.

You should also make sure to remove the `hierarchical` option from
`storage_manager` in `config/danbooru_local_config.rb` if you set it
there.

script/fixes/077_symlink_subdirectories.rb

@@ -0,0 +1,27 @@
+#!/usr/bin/env ruby
+
+require_relative "../../config/environment"
+
+def create_symlinks(dir)
+  FileUtils.mkdir_p(dir)
+
+  (0..255).each do |i|
+    subdir = "#{dir}/#{"%.2x" % i}"
+
+    if File.exist?(subdir)
+      puts "skipping #{subdir}"
+    else
+      puts "ln -sf . #{subdir}"
+      FileUtils.ln_sf(".", subdir)
+    end
+  end
+end
+
+root = Rails.root.join("public/data")
+
+create_symlinks(root)
+create_symlinks("#{root}/sample")
+create_symlinks("#{root}/preview")
+create_symlinks("#{root}/crop")
+
+FileUtils.ln_sf(".", "#{root}/original") unless File.exist?("#{root}/original")