feat: setup signal to consume transaction reveral events

[iloveagent57]

Description:

• Adds a reversed assignment state and reversed_at field.
• Sets up a signal handler that will consume the LEDGER_TRANSACTION_REVERSED openedx event and modify a related assignment record's state to reversed.

Jira:
ENT-9009

Testing

1. Run migrations
2. Have feat: emit transaction lifecycle events enterprise-subsidy#244 running (which implies that you have kafka etc running locally).
3. Ensure you have a reversible transaction related to some assignment.
4. In enterprise-access make app-shell and then run the consumer process with ./manage.py consume_events -t enterprise-subsidy-ledger-transaction-lifecycle --group_id enterprise_access_dev
5. Unenroll and refund the transaction from the enterprise-subsidy Transaction admin page.
6. Observe logs in the consumer process that the related assignment was reversed.
7. Observe in enterprise-access Django admin page for the assignment that the state has now changed to reversed.

Merge checklist:

• ./manage.py makemigrations has been run
  • Note: This must be run if you modified any models.
    • It may or may not make a migration depending on exactly what you modified, but it should still be run.

Post merge:

• Ensure that your changes went out to the stage instance
• Deploy to prod instance

[iloveagent57]

new pylint version made this a useless suppression.

[pwnage101]

a new era!