Delete unactivated users who sign up whose email address bounces

[alastair]

Now that we track bouncing emails (#884) we can see which new user accounts didn't receive their activation emails. Because they don't receive their emails we can be sure that they'll never be able to log in. We should proactively delete these accounts. We can have a cron script that runs after the cron A few things to consider if we do this:

• Do users often send support requests if they find that they didn't get an activation email, @bdejong? If we delete the accounts then we won't be able to help any more.
• If we delete accounts then the usernames become available again. Do we think that it's a problem that someone could notice that we "deleted their account"? I think that this isn't a huge problem

[philtgun]

One point made by @alastair in #1215 is that maybe we should wait 1 month before deleting unactivated users with bounced emails (for them to have time to contact support). I don't think it is important because users should be able to register again with correct email address.

Also as mentioned in #884 by me - about the users that have registered but haven't activated their account or logged in - maybe we want to delete those accounts also after some time (6 months or so). The way to get those users is to check for last_login=NULL with is_active=False indicating that user haven't activated their account.

Management command to cleanup unactivated users with bounced emails is implemented in #1189. Should we deal with inactive users in the same place as those are semantically related, or implement in separate management command?

@ffont, your thoughts?