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.

app/logical/storage_manager/local.rb

@@ -2,6 +2,14 @@
 class StorageManager::Local < StorageManager
   DEFAULT_PERMISSIONS = 0o644
 
+  attr_reader :base_dir
+
+  # @param base_url [String] the base directory where files are stored (ex: "/home/danbooru/public/data")
+  def initialize(base_dir: nil, **options)
+    @base_dir = base_dir.to_s
+    super(**options)
+  end
+
   def store(io, dest_path)
     temp_path = full_path(dest_path) + "-" + SecureRandom.uuid + ".tmp"
 
@@ -24,4 +32,10 @@ class StorageManager::Local < StorageManager
   def open(path)
     File.open(full_path(path), "r", binmode: true)
   end
+
+  protected
+
+  def full_path(path)
+    File.join(base_dir, path)
+  end
 end