Record cta click for in-product marketing emails

What does this MR do?

Since we now track the sent emails for the in-product marketing campaign, we also want to track when the CTA button in the email got clicked.

How to test locally:

• Sign in with root
• bin/rails c
  • Create the InProductMarketingEmail that gets created in the background job

    Namespaces::InProductMarketingEmail.create!(user: User.first, namespace: Group.first, track: 'create', series: 0)

  • Send the email

    NotificationService.new.in_product_marketing(User.first.id, Group.first.id, :create, 0)

• Go to http://localhost:3000/rails/letter_opener/
• In bin/rails c again
  • Check if cta_clicked_at was set

    Namespaces::InProductMarketingEmail.last
    # => [#<Namespaces::InProductMarketingEmail:0x000055839e63b9e8 id: 1, user_id: 1, namespace_id: 22, cta_clicked_at: Mon, 15 Mar 2021 15:28:18 UTC +00:00, track: "create", series: 0, created_at: Mon, 15 Mar 2021 15:27:35 UTC +00:00, updated_at: Mon, 15 Mar 2021 15:28:18 UTC +00:00>]

Related to:

• Issue: https://gitlab.com/gitlab-org/growth/team-tasks/-/issues/308
• Epic: gitlab-org/growth&62

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