Add option to delete blocked spammer accounts instead of just blocking

[sdeibel]

When a moderator blocks a spammer, the user account accumulates in the database with blocked status. Over time this creates thousands of dead accounts. This adds a DELETE_BLOCKED_USERS livesetting (default True) that deletes the user account entirely after content deletion, instead of just setting status to blocked.

When disabled, preserves the original set_status('b') behavior. The UI message says "deleted" or "blocked" accordingly.

[evgenyfadeev]

I think there might be some issues/improvements here:

• you might mark users as spammers by mistake - so perhaps it would be better to create a job that deletes these users after some grace period so that you can restore mistakenly deleted accounts before they are gone permanently.
• it may be worthwhile to store email addresses of blocked spammers - to reject re-registration with the same emails.
• if you delete the content immediately, there will be no spam samples to train the filter on - ideally the spam samples would be taken into separate storage before deleting the posts (this could be implemented later though - some loss of these samples could be tolerated of course)

So I would rather have this implemented as a delayed action, not immediate and with some related improvements.

[sdeibel]

I agree this probably needs more work, but the issue of thousands and thousands of spammer accounts has been a problem throughout for us, so I ended up going this far with it.