Armor should apply a bonus/malus to armor class rather than override "naked" AC

[anthonyronda]

An acceptable PR for this will:

• leverage the (not yet extant) database migration code to modify all world armor and packs armor to calculate a new bonus AC field calculated from the current AC/AAC fields, to be minimally disruptive to campaigns in action.

I'll update when the migration code is in place (see #32 and #151)

[engleback]

Maybe not change the database, just calculate the "armour bonus" as 9-DAC or AAC-10 (not sure about that, but you get the idea), and then use it