Add cancellation reason to /elections

[Bekabyx]

Ref #1972
Asana

Very straightforward one this.

To test locally:

• Make sure you're UTD on migrations
• Hit the EE API locally @ http://localhost:8000/api/elections/
• Elections returned should have a cancellation_reason field - this will currently be null for all elections because no migration has been done yet.

PR Checklist

Beta Give feedback

1. References a specific issue or if not, describes the bug or feature in detail
2. Note what card in Asana this work relates to
3. Instructions for how reviewers can test the code locally
4. Tests have been added and/or updated

[coveralls]

coverage: 67.92%. remained the same when pulling 845eaf1 on feature/add-cancellation-reason-to-api into 1e6fd0c on master.