[Boost] Use WordPress heartbeat API for slow polling

The Cloud CSS UI polls the back end for updates at two rates:
- Rapidly while CSS is being generated, to ensure updates are displayed quickly, and
- Slowly while CSS is not being generated to ensure UI consistency across browser tabs.

In the latter case, we should use the [WordPress heartbeat API](https://developer.wordpress.org/plugins/javascript/heartbeat-api/) to avoid adding unnecessary network traffic to users' Jetpack Boost settings page.

It still makes sense to actively poll while generation is ongoing, but any passive UI consistency polling should be done via WP Heartbeat.

Start with the Cloud CSS UI's two polling modes and read the linked WordPress Heartbeat API documentation. Preserve active polling during CSS generation, replace only passive UI-consistency polling with heartbeat updates, and verify that browser tabs still stay consistent without unnecessary requests.

Written by the indexing model from the issue text.