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/controllers/accounts/exports_controller_test.rb

@@ -0,0 +1,78 @@
+require "test_helper"
+
+class Account::ExportsControllerTest < ActionDispatch::IntegrationTest
+  setup do
+    sign_in_as :david
+  end
+
+  test "create creates an export record and enqueues job" do
+    assert_difference -> { Account::Export.count }, 1 do
+      assert_enqueued_with(job: ExportAccountDataJob) do
+        post account_exports_path
+      end
+    end
+
+    assert_redirected_to account_settings_path
+    assert_equal "Export started. You'll receive an email when it's ready.", flash[:notice]
+  end
+
+  test "create associates export with current user" do
+    post account_exports_path
+
+    export = Account::Export.last
+    assert_equal users(:david), export.user
+    assert_equal Current.account, export.account
+    assert export.pending?
+  end
+
+  test "create rejects request when current export limit is reached" do
+    Account::ExportsController::CURRENT_EXPORT_LIMIT.times do
+      Account::Export.create!(account: Current.account, user: users(:david))
+    end
+
+    assert_no_difference -> { Account::Export.count } do
+      post account_exports_path
+    end
+
+    assert_response :too_many_requests
+  end
+
+  test "create allows request when exports are older than one day" do
+    Account::ExportsController::CURRENT_EXPORT_LIMIT.times do
+      Account::Export.create!(account: Current.account, user: users(:david), created_at: 2.days.ago)
+    end
+
+    assert_difference -> { Account::Export.count }, 1 do
+      post account_exports_path
+    end
+
+    assert_redirected_to account_settings_path
+  end
+
+  test "show displays completed export with download link" do
+    export = Account::Export.create!(account: Current.account, user: users(:david))
+    export.build
+
+    get account_export_path(export)
+
+    assert_response :success
+    assert_select "a#download-link"
+  end
+
+  test "show displays a warning if the export is missing" do
+    get account_export_path("not-really-an-export")
+
+    assert_response :success
+    assert_select "h2", "Download Expired"
+  end
+
+  test "show does not allow access to another user's export" do
+    export = Account::Export.create!(account: Current.account, user: users(:kevin))
+    export.build
+
+    get account_export_path(export)
+
+    assert_response :success
+    assert_select "h2", "Download Expired"
+  end
+end