chore: Clean up report & knowledge base policies (#11234)

- Removes the portal_members table and all associated records
- Updates policies to use custom roles with knowledge_base_manage
permission
- Updates controllers, models, and views to work without portal
membership
- Adds tests for the new permission model

enterprise/app/controllers/enterprise/api/v2/accounts/reports_controller.rb

@@ -1,7 +0,0 @@
-module Enterprise::Api::V2::Accounts::ReportsController
-  def check_authorization
-    return if Current.account_user.custom_role&.permissions&.include?('report_manage')
-
-    super
-  end
-end