Add justfile command to automate download and setup of prod data in dev envs

[daveoconnor]

This is necessary for local dev to allow devs to have the github_username values that are on prod, in order to have an accurate representation of what the prod site will look like.

This will allow for quick setup of prod data using the urls mentioned in #1642.
Could be useful for initial project onboarding later, probably quicker and more complete than running the import scripts at a later date.