Instant Search: Add infinite scrolling

[jsnmoon]

Changes proposed in this Pull Request:

This enables paging via a "Load More" button with infinite scrolling support.

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

Yes, this adds paging to Jetpack Instant Search results.

Testing instructions:

• Add define( "JETPACK_SEARCH_PROTOTYPE", true ); to your wp-config.php.
• Ensure that your site has the Jetpack Pro plan and Jetpack Search enabled.
• Add a Jetpack Search widget to the Search page sidebar.
• Enter a query into the search widget.
• Ensure that the "Load more results" button renders like so:

• Scroll to the bottom of the page. Ensure that more results are fetched and appended to the list of search results.
• Without scrolling to the bottom, click on the "Load more results" button. Ensure that more results are fetched and appended to the page.

Proposed changelog entry for your changes:

None.

[gibrown]

I briefly contemplated resolving the conflicts but I though I think I see how the results are getting concatenated between requests, unsure I understand it well enough to do that merge. ;)

The scrolling works pretty well. Also tested when getting to the end of the results and that works well too.

Somehow though the filtering seems to be broken now. The initial search is not showing any filters. Sometimes they appear when scrolling, but then they disappear after applying some filtering. I assume it has to do with how the results are getting combined.

My other comments are pretty minor and not worth holding up this PR for. I like that you are directly relying on the CSS from the theme for the button. Looks good.

This was due to a silly mix up in a ternary statement -- should be fixed!

[gibrown]

Still has a bug somewhere. When I load more results the filters then disappear. I'm guessing that the aggs results are just getting overwritten by the second search query rather than persisted.

Aggregations should now be preserved for paged responses.

[gibrown]

Working for me now. Ship it!