Add users with deterministic username and password to development seed.

While developing, I want to see the system from the point of view of multiple users (often one on each browser). 

Like https://github.com/gitlabhq/gitlabhq/pull/5896, but that was declined because of the option, so no option here. Also this touches only users.

This is worth even without an option.

- we develop much more often than we take screenshots
- given enough faker users, the predictable ones will disappear