config: simplify mailer configuration again

Move everything to initializers, and replace the email settings
interceptor by a BalancerDeliveryMethod.

It has the advantage that it can be configured entirely from the
`config/environment.rb` file, without an extra file to look at.

spec/lib/balancer_delivery_method_spec.rb

@@ -0,0 +1,93 @@
+RSpec.describe BalancerDeliveryMethod do
+  class ExampleMailer < ApplicationMailer
+    def greet(name)
+      mail(to: "smtp_to", from: "smtp_from", body: "Hello #{name}")
+    end
+  end
+
+  class TestMail
+    def self.deliveries
+      @deliveries ||= []
+    end
+
+    def self.deliveries=(val)
+      @deliveries = val
+    end
+
+    attr_accessor :settings
+
+    def initialize(values)
+      @settings = values.dup
+    end
+
+    def deliver!(mail)
+      Mail::CheckDeliveryParams.check(mail)
+      self.class.deliveries << mail
+    end
+  end
+
+  class MockSmtp < TestMail; end
+
+  class MockSendmail < TestMail; end
+
+  class FixedSequence
+    def initialize(sequence)
+      @enumerator = sequence.each
+    end
+
+    def rand(_)
+      @enumerator.next
+    end
+  end
+
+  before do
+    ActionMailer::Base.add_delivery_method :mock_smtp, MockSmtp
+    ActionMailer::Base.add_delivery_method :mock_sendmail, MockSendmail
+    ActionMailer::Base.add_delivery_method :balancer, BalancerDeliveryMethod
+
+    ExampleMailer.delivery_method = :balancer
+  end
+
+  context 'when a single delivery method is provided' do
+    before do
+      ActionMailer::Base.balancer_settings = { mock_smtp: 10 }
+    end
+
+    it 'sends emails to the selected delivery method' do
+      mail = ExampleMailer.greet('Joshua').deliver_now
+      expect(mail).to have_been_delivered_using(MockSmtp)
+    end
+  end
+
+  context 'when multiple delivery methods are provided' do
+    before do
+      ActionMailer::Base.balancer_settings = { mock_smtp: 10, mock_sendmail: 5 }
+
+      rng_sequence = [3, 14, 1]
+      BalancerDeliveryMethod.random = FixedSequence.new(rng_sequence)
+    end
+
+    after do
+      BalancerDeliveryMethod.random = Random.new
+    end
+
+    it 'sends emails randomly, given the provided weights' do
+      mail1 = ExampleMailer.greet('Lucia').deliver_now
+      expect(mail1).to have_been_delivered_using(MockSmtp)
+
+      mail2 = ExampleMailer.greet('Damian').deliver_now
+      expect(mail2).to have_been_delivered_using(MockSendmail)
+
+      mail3 = ExampleMailer.greet('Rahwa').deliver_now
+      expect(mail3).to have_been_delivered_using(MockSmtp)
+    end
+  end
+
+  # Helpers
+
+  def have_been_delivered_using(delivery_class)
+    satisfy("have been delivered using #{delivery_class}") do |mail|
+      delivery_class.deliveries.include?(mail)
+    end
+  end
+end