Add "data export" feature

- Adds a button in Account Settings where you can request a ZIP export of your
  Fizzy data
- Export files are created in the background. When ready, a link to
  download them is sent to the requester.
- Exports expire after 24 hours. And are limited to 10 per day.

test/mailers/export_mailer_test.rb

@@ -0,0 +1,16 @@
+require "test_helper"
+
+class ExportMailerTest < ActionMailer::TestCase
+  test "completed" do
+    export = Account::Export.create!(account: Current.account, user: users(:david))
+    email = ExportMailer.completed(export)
+
+    assert_emails 1 do
+      email.deliver_now
+    end
+
+    assert_equal [ "david@37signals.com" ], email.to
+    assert_equal "Your Fizzy export is ready", email.subject
+    assert_match %r{/exports/#{export.id}}, email.body.encoded
+  end
+end

test/mailers/previews/export_mailer_preview.rb

@@ -0,0 +1,11 @@
+class ExportMailerPreview < ActionMailer::Preview
+  def completed
+    export = Account::Export.new(
+      id: "preview-export-id",
+      account: Account.first,
+      user: User.first
+    )
+
+    ExportMailer.completed(export)
+  end
+end