Search: Allow search results to have individual comments rather than just posts

Currently we limit to only matching on the 100 most recent comments.

The request (see below) is really to have individual comments be the records that are returned as search results instead of treating a comment as a part of the post. This will require some expanded version of https://github.com/Automattic/jetpack/issues/13927 to be implemented and some controls for the end user to be able to select what is being searched against by default.

This also raises questions about how other sites would feel about how much this would increase their record count and thus their prices. Which probably means we would also need to implement https://github.com/Automattic/jetpack/issues/13295 for it to be palatable.

As an interim solution we could raise the 100 recent comment threshold, but that seems unlikely to actually help here.

Start by reviewing the expanded approach described in issue 13927 and the record-count approach in issue 13295. The issue provides no files, tests, or entry points; done would require individual comments as searchable records, user controls for the default search scope, and an acceptable record-count impact for sites.

Written by the indexing model from the issue text.