GitLab.com only newsletter opt-in at signup

Description

Due to laws in various countries, it is not possible to automatically opt everyone into marketing communications when they sign up to GitLab.com.

Proposal

1. Add a checkbox to the signup process to receive updates and marketing communication from GitLab
2. This checkbox only appears on gitlab.com
3. The checkbox should un-checked by default
4. When submitting the event that is picked up by our marketing platform, we need to include this piece of metadata in the payload

Links / references

https://gitlab.com/gitlab-com/marketing/issues/931 https://gitlab.com/gitlab-com/support-forum/issues/1616#note_25419026