Test: task service tests; minimal test infrastructure for other services

[jfmcquade]

PR Checklist

• [x] PR title descriptive (can be used in release notes)

Description

Follow-up to #2176

• Adds unit tests for the task service
• Includes some minor refactors of task logic informed by writing the tests
• Removes task_group_set_highlighted action
  • This was unused and relies on "skipped_field" column in tasks, which was never implemented
  • The logic remains, commented out, for skipping ahead and forward within a group of tasks by explicitly setting the highlighted task
• Adds test scaffolding and mocks for other services on which the task service depends:
  • campaign
  • template field
  • template translate
  • app config
  • app data
• Includes #2080
  • This PR has now been closed, picked up in #2196

Git Issues

Closes #

Screenshots/Videos