Features: User fixture

This introduces a very simple fixture graph that should be maintainable (one of Login, Account, User, and Idea).

Spec suite before:

- Finished in 41.49 seconds (seed 54307)
- Finished in 41.39 seconds (seed 28768)

After:

- Finished in 35.24 seconds (same seeds)
- Finished in 31.99 seconds

Gain: **18.8%**