feat(EMS-4035-4042): declarations - modern slavery - data migration

[ttbarnes]

⚠️ WIP

⚠️ TODO

• Data structure has changed since this was created.
• Fix foreign key constraint issue.
• Update README.

Introduction ✏️

This PR adds a data migration script so that when main-declarations-modern-slavery is deployed, we can migrate previous applications data structure into the new data structure required for "declarations - modern slavery".

Resolution ✔️

• Create data migration V3 to V4 index script.
• Create migration helper functions:
  • getAllDeclarationModernSlaveries
  • getAllDeclarations
• Move getAllSubmittedApplications and createCuid into a new parent helpers directory to be DRY.
• Create various migration functions so that when run, the following happens:
  • Application table - migratedFrom the V2->V3 field is replaced with a V3->V4 field.
  • A new DeclarationModernSlavery table is created.
  • A new DeclarationModernSlaveryVersion table is created.
  • For each existing declaration row (regardless of the application status), create DeclarationModernSlavery and DeclarationModernSlaveryVersion rows.
  • The Declaration table is updated appropriately.
  • Keys and constraints are added appropriately to the new tables.

Miscellaneous ➕

• Minor documentation fixes/improvements.