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.

test/test_helper.rb

@@ -49,9 +49,9 @@ class ActiveSupport::TestCase
     Socket.stubs(:gethostname).returns("www.example.com")
 
     @temp_dir = Dir.mktmpdir("danbooru-temp-")
-    storage_manager = StorageManager::Local.new(base_dir: @temp_dir)
+    storage_manager = StorageManager::Local.new(base_url: "https://www.example.com/data", base_dir: @temp_dir)
     Danbooru.config.stubs(:storage_manager).returns(storage_manager)
-    Danbooru.config.stubs(:backup_storage_manager).returns(StorageManager::Null.new)
+    Danbooru.config.stubs(:backup_storage_manager).returns(StorageManager::Null.new(base_url: "/", base_dir: "/"))
   end
 
   teardown do