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

spec/factories/portal_members.rb

@@ -1,6 +0,0 @@
-FactoryBot.define do
-  factory :portal_member do
-    portal
-    user
-  end
-end