feat(EMS-3828-3876): currency of agent charges - save and back, improve e2e assertions

[ttbarnes]

Introduction ✏️

This PR adds "save and back" functionality to the "export contract - currency of agent charges" form.

Previously, the common currency forms did not have "save and back" functionality.

Therefore, as part of this feature, the DRY E2E currency form assertion functions needed some refactoring to support DRY "save and back" tests.

Resolution ✔️

• Update the E2E currency form assertion function (formSubmissionAssertions) so that:
  • It can be executed with 1x line of code, formSubmission({}).executeTests();
  • It handles high level describe blocks (instead of repeating in individual tests.
  • It tests "via continue/submit" and "via save/back" button scenarios.
  • The main file is now an index file, with previously existing functions extracted into their own files.
• Update assertCurrencyFormFields to conditionally click submit or "Save and back" buttons.
• Rename completeNonCurrencyFields to completeNonCurrencyFieldsFunction.
• Update E2E tests.
• Create "save and back" UI POST routes for "export contract - currency of agent charges"
• Update "your buyer - currency of late payments" controllers to have a page variables funtion with SAVE_AND_BACK_URL.

Miscellaneous ➕

• Add various missing documentation.
• Fix some typos.