Add admin API for managing canonical e-mail blocks (#19067)

db/migrate/20220827195229_change_canonical_email_blocks_nullable.rb

@@ -0,0 +1,5 @@
+class ChangeCanonicalEmailBlocksNullable < ActiveRecord::Migration[6.1]
+  def change
+    safety_assured { change_column :canonical_email_blocks, :reference_account_id, :bigint, null: true, default: nil }
+  end
+end