Determine staff members based on flag

This replaces the old system based on email addresses, the aim is to make it more generic and thus appropriate for OSS/self-hosting

See: https://3.basecamp.com/2914079/buckets/37331921/todos/9302705265#__recording_9320051455

db/migrate/20251125130010_add_a_staff_flag_to_identities.rb

@@ -0,0 +1,5 @@
+class AddAStaffFlagToIdentities < ActiveRecord::Migration[8.2]
+  def change
+    add_column :identities, :staff, :boolean, null: false, default: false
+  end
+end