Add rake task to help generate preparation MRs

What

• Creates bundle exec rake "patch_merge_request[X.Y.Z]" rake task to create a merge request from the preparation_merge_request.md.erb template
• Currently based on the !162 (merged) branch, but needs rebasing once that merges as well as to work with updated code
• Updates instructions in picking-into-merge-requests.md as well as in monthly and patch templates.

Why

Copying previous descriptions and updating branch names / repos was tedious, this automates generating the description and can be extended to create the MRs automatically.

Related

Closes https://gitlab.com/gitlab-org/release-tools/issues/98