Ignore ApplicationWorkHistoryBreak#application_form_id

[CatalinVoineag]

Context

We don't need the application_form_id on the
application_work_history_break table, we use the breakable column to
link it to an application form.

This commit ignores the application_form_id and makes sure the deletion
of the history break is going to happen when deleting a candidate, just
like before.

We made application_form_id nullable here
9d60480

Changes proposed in this pull request

Ignore application_form_id
Delete application_work_history_break when the candidate is deleted. Cascading deletes don't work with polymorphic relations.

Guidance to review

Things to check

• If the code removes any existing feature flags, a data migration has also been added to delete the entry from the database
• This code does not rely on migrations in the same Pull Request
• If this code includes a migration adding or changing columns, it also backfills existing records for consistency
• If this code adds a column to the DB, decide whether it needs to be in analytics yml file or analytics blocklist
• API release notes have been updated if necessary
• If it adds a significant user-facing change, is it documented in the CHANGELOG?
• Required environment variables have been updated added to the Azure KeyVault
• Inform data insights team due to database changes
• Make sure all information from the Trello card is in here
• Rebased main
• Cleaned commit history
• Tested by running locally
• Add PR link to Trello card

[CatalinVoineag]

Edit work experiences - Change db structure to accommodate work experience edits