Instant Search: Account for site URL when appending query string

[jsnmoon]

Fixes #13845.

Changes proposed in this Pull Request:

• Propagates output of site_url() to Instant Search's front-end application.
• Uses the URL API for appending query strings.
• Imports url-polyfill package to ensure IE 11 support.

Testing instructions:

• Testing on a WordPress installation at a root URL (e.g. /):

  1. Ensure that you've set up Jetpack Search on your WordPress installation.
  2. Enter text into your Jetpack Search input.
  3. Ensure that your URL is correctly formatted (e.g. /?s=hello)

• Testing on a WordPress installation at a non-root URL (e.g. /wp-blog):

  1. Ensure that you've set up Jetpack Search on your WordPress installation.
  2. Enter text into your Jetpack Search input.
  3. Ensure that your URL is correctly formatted (e.g. /wp-blog/?s=hello)

Setting up Jetpack Search:

1. 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.

2. 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).

3. 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.

Proposed changelog entry for your changes:

• No changelog necessary. Changelog in Instant Search: Update prototype with downstream changes #13761 should be sufficient.

[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 3671d1d

[gibrown]

Works well in almost all cases that I tested (tested on my subdirectory install of WP). The only case I found was when using the back button it didn't reload the original page. For instance, load the homepage and then do a single filter, click back, and although the url gets correctly updated the search results do not get cleared.

We could potentially break this out into a separate issue if it is tough to fix, it is a pretty minor corner case.

Updated to use home_url() instead of site_url()

[jsnmoon]

The only case I found was when using the back button it didn't reload the original page.

This should be fixed now; I added handling for popstate events, which are triggered by history.back() and history.forward().

[jeherve]

LGTM. Merging.