Instant Search: Use href to restore previous location state

[jsnmoon]

Fixes #13808.

Changes proposed in this Pull Request:

• Removes reliance on site_url() from PHP.
• Saves window.location.href on page load for future use.
• When hiding search results, the initial href on page load is restored onto window.location via the History API.

Is this a new feature or does it add/remove features to an existing part of Jetpack?

• No.

Testing instructions:

• Add define( "JETPACK_SEARCH_PROTOTYPE", true ); to your wp-config.php.
  If using Jetpack's Docker development environment, you can create a file at /docker/mu-plugins/instant-search.php and add the define there.

• Ensure that your site has the Jetpack Pro plan and Jetpack Search enabled.
  You can enable Jetpack Search in the Performance tab within the Jetpack menu (/wp-admin/admin.php?page=jetpack#/performance).

• Select a theme of your choosing and add a Jetpack Search widget to the site via the customizer, preferably with some filters enabled. If you're using a theme with a sidebar widget area, please add the Jetpack Search widget there.

• Navigate to a URL with a query parameter in the URL, like ?testQuery=true.

• Perform a search by entering some text into the search input box. Ensure that a query string value for your search term has been appended onto the URL (e.g. /?testQuery=true&s=some-search-value).

• Hide the search interface by deleting the text in the search input box. Ensure that the URL has been restored to /?testQuery=true

Proposed changelog entry for your changes:

• None.

[jetpackbot]

Thank you for the great PR description!

When this PR is ready for review, please apply the [Status] Needs Review label. If you are an a11n, please have someone from your team review the code if possible. The Jetpack team will also review this PR and merge it to be included in the next Jetpack release.

Scheduled Jetpack release: November 5, 2019.
Scheduled code freeze: October 29, 2019

Generated by 🚫 dangerJS against dd54001

[bluefuton]

Works well for me! I had one question about removing siteUrl, but otherwise looks good to go. 🚢

[jeherve]

This tests well. Merging.