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

spec/fabricators/canonical_email_block_fabricator.rb

@@ -0,0 +1,4 @@
+Fabricator(:canonical_email_block) do
+  email "test@example.com"
+  reference_account { Fabricate(:account) }
+end