GDPR Article 15 Workflow Updates

Added a bit more detail to this workflow and moved the structure around so that it flows smoother (imo).

I changed the header on the final response to requests submitted through gdpr-request@gitlab.com from Email template for successfully deleted accounts to Request completed response because it didn't seem like all of our article 15 requests will also request the deletion of an account. At least, our first one didn't.

Happy to change that back if that's incorrect.