Add or update mailman3 logrotate configs

Hi,

The package currently ships out the following:

| ubuntu@juju-c4acfb-prod-lists-canonical-com-57:/etc/logrotate.d$ dpkg-query -L mailman3 | grep logrot
| /etc/logrotate.d
| /etc/logrotate.d/mailman3
| ubuntu@juju-c4acfb-prod-lists-canonical-com-57:/etc/logrotate.d$ cat /etc/logrotate.d/mailman3
| /var/log/mailman3/mailman.log {
| daily
| rotate 5
| compress
| delaycompress
| missingok
| notifempty
| create 640 list list
| postrotate
| if /etc/init.d/mailman3 status >/dev/null; then \
| /usr/bin/mailman reopen >/dev/null; \
| fi;
| endscript
| }

We should update this so retention is configurable and also include the others (bounce.log, debug.log, and smtp.log):

| ubuntu@juju-c4acfb-prod-lists-canonical-com-57:~$ ls -la /var/log/mailman3/
| total 13885696
| drwxr-xr-x 2 list list 4096 Mar 27 06:25 .
| drwxrwxr-x 15 root syslog 12288 Mar 27 06:25 ..
| -rw-rw---- 1 list list 1108809 Mar 27 16:25 bounce.log
| -rw-rw---- 1 list list 0 Jun 11 2018 debug.log
| -rw-r----- 1 list list 43769060 Mar 28 01:45 mailman.log
| -rw-r----- 1 list list 50108755 Mar 27 06:24 mailman.log.1
| -rw-r----- 1 list list 3402823 Mar 26 06:25 mailman.log.2.gz
| -rw-r----- 1 list list 3917180 Mar 25 06:24 mailman.log.3.gz
| -rw-r----- 1 list list 4020020 Mar 24 06:25 mailman.log.4.gz
| -rw-r----- 1 list list 4686363 Mar 23 06:24 mailman.log.5.gz
| -rw-rw---- 1 list list 14107871952 Mar 28 01:45 smtp.log
| ubuntu@juju-c4acfb-prod-lists-canonical-com-57:~$