Move $sa_spam_subject_tag and $sa_local_tests_only to the Maia system configuration table

[rjl]

It is more convenient in many cases for an administrator to set configuration options from Maia's System Configuration page, which stores its values in the database, than to have to edit the amavisd.conf file. As with some of the other settings that have been migrated to the database, this simply involves looking them up at runtime within amavisd-maia.

[dmorton]

amavis currently looks this up up per user around 8449. we could just add a column to the policy table, I think, and let each user call it what they want...