Add opt-in "I'd like to receive updates via email" checkbox

What does this MR do?

Adds the checkbox that allows users to opt-in to receive email updates from GitLab.com.

Are there points in the code the reviewer needs to double check?

https://gitlab.com/gitlab-com/marketo-tools/merge_requests/7 depends on this MR.

Why was this MR needed?

Legal and ethical reasons.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #2447 (closed)