Add user setting for opting into marketing emails

What does this MR do?

This adds the user setting Receive product marketing emails for opting into marketing emails.

This is related to gitlab-org/growth&62

Screenshot

doc/user/profile/notifications.md

@@ -56,6 +56,8 @@ Your **Global notification settings** are the default settings unless you select
   - This is the email address your notifications are sent to.
 - Global notification level
   - This is the default [notification level](#notification-levels) which applies to all your notifications.
+- Receive product marketing emails
+  - Check this checkbox if you want to receive periodic emails related to GitLab features.
 - Receive notifications about your own activity.
   - Check this checkbox if you want to receive notification about your own activity. Default: Not checked.