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/models/canonical_email_block_spec.rb

@@ -0,0 +1,47 @@
+require 'rails_helper'
+
+RSpec.describe CanonicalEmailBlock, type: :model do
+  describe '#email=' do
+    let(:target_hash) { '973dfe463ec85785f5f95af5ba3906eedb2d931c24e69824a89ea65dba4e813b' }
+
+    it 'sets canonical_email_hash' do
+      subject.email = 'test@example.com'
+      expect(subject.canonical_email_hash).to eq target_hash
+    end
+
+    it 'sets the same hash even with dot permutations' do
+      subject.email = 't.e.s.t@example.com'
+      expect(subject.canonical_email_hash).to eq target_hash
+    end
+
+    it 'sets the same hash even with extensions' do
+      subject.email = 'test+mastodon1@example.com'
+      expect(subject.canonical_email_hash).to eq target_hash
+    end
+
+    it 'sets the same hash with different casing' do
+      subject.email = 'Test@EXAMPLE.com'
+      expect(subject.canonical_email_hash).to eq target_hash
+    end
+  end
+
+  describe '.block?' do
+    let!(:canonical_email_block) { Fabricate(:canonical_email_block, email: 'foo@bar.com') }
+
+    it 'returns true for the same email' do
+      expect(described_class.block?('foo@bar.com')).to be true
+    end
+
+    it 'returns true for the same email with dots' do
+      expect(described_class.block?('f.oo@bar.com')).to be true
+    end
+
+    it 'returns true for the same email with extensions' do
+      expect(described_class.block?('foo+spam@bar.com')).to be true
+    end
+
+    it 'returns false for different email' do
+      expect(described_class.block?('hoge@bar.com')).to be false
+    end
+  end
+end