When adding a service contact in the middle of a case, we should default to *not* using master list

[nonprofittechy]

see #60

Currently when you add a service contact in the middle of the filing interview, you only see service contacts on the master list. It seems that the non-master list option is intentionally designed for this use case, but it's a bit hard to use in our current procedure.

The contact can still be retrieved though, by typing or copying and pasting the GUID for the service contact instead of using the picklist, but it's not ideal. Probably safe to defer any refactoring until post-certification; will leave it on the certification milestone but ready to be triaged.