FEATURE: Add dark mode option for category backgrounds (#24003)

Adds a new upload field for a dark mode category background that will be used as an alternative when Discourse is using a dark mode theme.

db/migrate/20231018225833_add_dark_mode_background_to_categories.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+class AddDarkModeBackgroundToCategories < ActiveRecord::Migration[7.0]
+  def change
+    add_column :categories, :uploaded_background_dark_id, :integer, index: true
+  end
+end