Add a kludgy 2-step user deletion process to the admin

[jnm]

Please review #2149 first.

This closes kobotoolbox/tasks#32. It works from the Django admin user list only. Trying to delete an individual user from the admin change form will still raise a 500 error.

This isn't great, but let's put our energy toward having separate databases for KoBoCAT and KPI.

[noliveleger]

@jnm. I know it's a kludgy 2-step but I'm wondering if we could (at least) deactivate log in as user after using step1. Or just adding a column which display users' current state of deletion. (e.g. Step 1 deletion - done).

I deleted a bunch of users with quite the same usernames. I had to do something between step-1 and step-2. I had a hard time to remember which ones I wanted to delete.

You probably think admin users would not make a pause between 2 steps. You maybe right.

[jnm]

I think I understand why it's a bad workflow, but I'm not sure how deactivating log in as user would help. Could you clarify that, unless you think what I'm about to write would work well enough? :)

What if, after Remove related objects..., we redirected back to a filtered view of users that displayed only the ones whose related objects had just been removed? For example, if we checked off users whose PKs were 1 through 7, clicked Removed related objects..., and confirmed the action, we would then be taken to https://kf.kobotoolbox.org/admin/auth/user/?pk__in=1,2,3,4,5,6,7, which would display only those users. Maybe there's even a way we could have the checkboxes be automatically ticked.