Add setting for enabling in product marketing emails

What does this MR do?

This adds the admin setting Enable in product marketing emails for enabling in product marketing emails on an instance level.

Screenshot

Migration output

╰─ rails db:migrate:redo
== 20210216122140 AddInProductMarketingEmailsEnabledSetting: reverting ========
-- remove_column(:application_settings, :in_product_marketing_emails_enabled, :boolean, {:null=>false, :default=>true})
   -> 0.0113s
== 20210216122140 AddInProductMarketingEmailsEnabledSetting: reverted (0.0137s)

== 20210216122140 AddInProductMarketingEmailsEnabledSetting: migrating ========
-- add_column(:application_settings, :in_product_marketing_emails_enabled, :boolean, {:null=>false, :default=>true})
   -> 0.0064s
== 20210216122140 AddInProductMarketingEmailsEnabledSetting: migrated (0.0065s)

This is related to:

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