Navigating away from Audiences modal Add / Edit doesn't cause warning

Related to humanmade/altis-analytics#96 and spinning off of #147 the same issue described in humanmade/altis-analytics#96 applies in the Audiences modal, where you can navigate away and not save your changes or be prompted that changes were made.

Steps to reproduce:

1. Create or edit a post
2. Add a personalization block
3. Create a variant
4. Click to add an audience
5. Create or Edit an audience
6. Make some changes
7. Navigate away from the page (close the modal or click "Back to Audiences" button)
8. Lose all modifications made to audience, thinking it has all updated and been saved