SPIKE: move rake tasks business logic and leave tasks as executors

[scruti]

Trello card URL

Changes in this PR:

Started with the existing backfilling tasks as an example.

The intention

I am trying to find a normalised way to approach our one-off/regular manually executed tasks/scripts for our service, given the recent pain caused by coverage tools struggling to play ball with Rake tasks logic testing.

The approach

• Move the logic of the tasks to whatever ruby object is appropriate:
  • Service objects for "commands" (again, service objects are used a bit of a "stuff drawer")
  • Jobs for things to execute in the background.
  • etc.
• Proper unit code testing of the logic in the ruby code, without Rake quirky issues
• The rake task is left as a mere executor of the ruby code. And the task is namespaced in its appropriate area (EG: data.rake)

Data & Schema Changes

If this PR modifies data structures or validations, check the following:

• Adds/removes model validations
• Adds/removes database fields
• Modifies Vacancy enumerables (phases, working patterns, job roles, key stages, etc.)

If any of the above options has changed then the author must check/resolve all of the following...

Integration Impact

Does this change affect any of these integrations?

• DfE Analytics platform
• Legacy imports mappings
• DWP Find a Job export mappings
• Publisher ATS API (may require mapping updates or API versioning)

User Experience & Data Integrity

Could this change impact:

• Existing subscription alerts (will legacy subscription search filters break?)
• Legacy vacancy copying (will copied vacancies fail new validations?)
• In-progress drafts for Vacancies or Job Applications