Add canonical e-mail blocks for suspended accounts (#16049)

Prevent new accounts from being created using the same underlying
e-mail as a suspended account using extensions and period
permutations. Stores e-mails as a SHA256 hash

db/migrate/20210416200740_create_canonical_email_blocks.rb

@@ -0,0 +1,10 @@
+class CreateCanonicalEmailBlocks < ActiveRecord::Migration[6.1]
+  def change
+    create_table :canonical_email_blocks do |t|
+      t.string :canonical_email_hash, null: false, default: '', index: { unique: true }
+      t.belongs_to :reference_account, null: false, foreign_key: { on_cascade: :delete, to_table: 'accounts' }
+
+      t.timestamps
+    end
+  end
+end