Seed dev database with massive amount of Users, Projects and its relations

What does this MR do?

Prepares the seed_fu rake task to insert massive amount of records for Users, Projects and its relations. The goal here is:

1. Setting a standard for making these insertions for seed_fu rake, so more people can contribute
2. With these records in place, it'll be easier for catching performance issues during development

The "mass inserted" projects only create the barebones in order to make the projects accessible:

• Routes
• Namespaces
• Project feature

Closes #17211 (closed)

Follow up with seed dump proposal: #32098