convert external services tests to use new db mocks

Created by: camdencheek

This PR converts the tests in external_services_test.go to use the new database mocks.

I'm working towards creating a critical mass of example tests using the new database mocks so the codebase moves in that direction independently of my intervention. I'll keep making incremental progress towards deprecating the global mocks, but I expect the PRs to slow down a bit from here, both because database.DB is spread through the codebase enough that I can use the new mocks for new tests, and because I expect to have less slack time moving forward.