FEATURE: Support designating multiple groups as mods on category (#28655)

Currently, categories support designating only 1 group as a moderation group on the category. This commit removes the one group limitation and makes it possible to designate multiple groups as mods on a category.

Internal topic: t/124648.

db/migrate/20240828191047_create_category_moderation_groups.rb

@@ -0,0 +1,21 @@
+# frozen_string_literal: true
+
+class CreateCategoryModerationGroups < ActiveRecord::Migration[7.1]
+  def change
+    create_table :category_moderation_groups do |t|
+      t.integer :category_id
+      t.integer :group_id
+
+      t.timestamps
+    end
+
+    add_index :category_moderation_groups, %i[category_id group_id], unique: true
+
+    execute <<~SQL
+      INSERT INTO category_moderation_groups (category_id, group_id, created_at, updated_at)
+      SELECT id, reviewable_by_group_id, CURRENT_TIMESTAMP, CURRENT_TIMESTAMP
+      FROM categories
+      WHERE reviewable_by_group_id IS NOT NULL
+    SQL
+  end
+end