Add a script to seed a project configured for merge trains

What does this MR do and why?

This adds a script for developers/designers to use in order to add a configured merge trains project with 20 MRs to their gdk.

To run the script:

rake gitlab:seed:merge_trains:project

It doesn't have an issue. I just started taking some notes to get projects configured more quickly but decided to share with a rake script. I don't think it needs tests since it's not customer critical.

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