Add "Without sign-in last 30 days" and "Without sign-in" filters

Adds two new filters for the `admin/users` page:

* Without sign-in last 30 days
* Without sign-in

![image](https://cloud.githubusercontent.com/assets/1192780/4034670/a7736f0e-2c87-11e4-8391-cba14b8291ef.png)

Ideally I'd like to have some advanced filters and sort options like the `admin/projects` page but this is an improvement. It'd be great if we tracked when each user last committed (e.g. `last_commited_at`) or completed an activity (e.g. `last_activity_at`) such as commenting on an issue so we could gauge how active users are.

I'll squash commits and add a meaningful commit message later.
If others have any ideas how to search for users with `current_sign_in_at: nil` OR `current_sign_in_at` > `30.days.ago` let me know. This is the best I could get to work.