Add process for "Pick into Backports" MRs. Update preparation MR template steps.

What

• Updates steps in templates/preparation_merge_request.md.erb and includes checklist
• Includes cherry-picking Pick into Backports MRs in those steps
• Currently targets fake master branch to show only changes since https://gitlab.com/gitlab-org/release-tools/merge_requests/173 until that merges

Why

Unclear process for when to pick MRs labeled Pick into Backports means they could easily get missed at the moment.

Related

• Changes to gitlab-ce/PROCESS.md in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13565
• https://gitlab.com/gitlab-org/release-tools/issues/107