Move mailer preview to app directory

What does this MR do?

• We move spec/mailers/previews to app/mailers/previews

CE at: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20430

Relevant docs, https://docs.gitlab.com/ee/development/emails.html#mailer-previews

Why was this MR needed?

It doesn't make sense to use something from spec/ in development

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Conform by the code review guidelines
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer

What are the relevant issue numbers?

See #5448 (closed)