Create ability to moderate users

There are user cases where it might be appropriate to have a site open to (public) registration, but where the existing `ckan.auth.*` functionality doesn't quite meet the needs.

Rather than have an external form or process for managing the users (e.g., re-route `/user/register` to a third-party form), it might be nice to do this in ckan.

```
2014-12-03 18:14:09 < amln> it kinda makes me think 'states' of users
2014-12-03 18:14:47 < amln> (pending, active, suspended, deleted, purged)
2014-12-03 18:16:20 < amln> but i think we've had some discussion in ckan terms under password management / resets.
```

Start by reviewing CKAN's existing ckan.auth functionality and the /user/register flow, then check the related password management and reset discussions mentioned in the issue. Clarify the proposed user states—pending, active, suspended, deleted, and purged—and define the moderation actions and registration behavior that would constitute completion.

Written by the indexing model from the issue text.