fix: infinite reload bug and exit race condition

[angeloashmore]

A new PR will be opened that ports these changes without the style commits. Style changes (using Prettier + ESLint) will be moved to a separate PR.

---

This PR addresses two related issues:

1. Previews can sometimes get into a state where the page refreshes endlessly. See Bug: Prismic toolbar new tab refresh loop. #66 for details and how to reproduce the issue.
2. When two preview windows for the same session are open, and one is exited, the other window will re-enable previews. This happens because the non-exited window is checking for preview updates on an interval and upserts (i.e. re-enables) the preview cookie that was deleted by the exiting window.

These issues are resolved by treating the preview state from the prismic.io iframe as the source of truth.

• Before starting a preview session, check if a ref is present in the latest preview state. If not, there is no active preview session. The toolbar does not need to refresh the page.
• On each interval where the toolbar checks for preview updates, first check if the preview cookie exists. If it does not, exit the preview.

This has the nice side-effect of exiting previews across all windows at once from a single toolbar "X" click.

[angeloashmore]

Closing in favor of #97.