Allow customizing agent type

[maxpatiiuk]

Requested by @grantfitzsimmons

Considerations:
Agent type is hard coded in sp6 and sp7.
We could allow people to override it in sp7 if they are willing to accept it will break sp6 compatibility.
There are some business rules that rely on agent types: agent_types_other_and_group_do_not_have_addresses
Front-end in sp7 hard codes agent type pick list, but only in one place. Beyond that it doesn't care what agent types exist, thus can be refactored to fetch agent type pick list from back-end (note: agent type pick list does not exist in the database at the moment - it's purely hardcoded)
Back-end has logic for agent types for workbench. That could should be refactored to treat agent type like any other system, read-only pick list.
There is also a question of default value. Workbench assigns agent type 1 by default (Person). if agent type pick list is customizable, and someone reassigns or deletes Person, things will be broken.
Beyond that, back-end also hard codes the agent type pick list and can be refactored to fetch pick list from the database