Mailing detail report does not report bounces correctly

The Mailing Detail report allows you to view and filter by the 'delivery status' of a mailing.

There were three issues:
1) The status was not consistently capped
2) Filtering by 'successful' also displays emails that 'bounced' later
3) Filtering by 'bounced' shows emails with the status 'successful'

This is down to two errors in the logic:

1) It assumes that if a mailing was successfully delivered, it did not bounce later (ie: gave it the status of 'successful', even though it bounced)
2) It did not exclude mailings that appear in the 'bounce' table when filtering by 'successful' mailings

We think the patch below fixes the issue. Even though a mailing was 'successfully' delivered, it should still have a status of 'bounced' in the report if it was later reported to be a bounce.

Joint credit to Alex C for this one.