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

db/migrate/20250402233933_remove_portal_members.rb

@@ -0,0 +1,16 @@
+class RemovePortalMembers < ActiveRecord::Migration[7.0]
+  def up
+    drop_table :portal_members
+  end
+
+  def down
+    create_table :portal_members do |t|
+      t.references :portal, index: false
+      t.references :user, index: false
+      t.timestamps
+    end
+
+    add_index :portal_members, [:portal_id, :user_id], unique: true
+    add_index :portal_members, [:user_id, :portal_id], unique: true
+  end
+end