storage manager: refactor base_dir option.

Fix it so the `base_dir` option is only required by subclasses that
actually use it. The StorageManager::Mirror class doesn't use it.

test/test_helper.rb

@@ -51,7 +51,7 @@ class ActiveSupport::TestCase
     @temp_dir = Dir.mktmpdir("danbooru-temp-")
     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(base_url: "/", base_dir: "/"))
+    Danbooru.config.stubs(:backup_storage_manager).returns(StorageManager::Null.new)
   end
 
   teardown do