Build and serve wpcom-block-editor inside Jetpack

Currently, wpcom-block-editor is loaded from widgets.wp.com.

In its current form, for _most_ Jetpack sites, it serves two features:
- switch to classic editor button (powered by jQuery, which as a side-note seems an odd fit for a React component)
- some rich text buttons for underline and justify

At a minimum, this file should be built in Jetpack and served from the origin. Separately, we should evaluate if it's necessary at all for non-Atomic Jetpack sites.

Start by locating wpcom-block-editor and the code that currently loads it from widgets.wp.com. Trace how Jetpack builds and serves related editor assets, then verify that this component is served from the site origin and document whether it is still needed for non-Atomic sites.

Written by the indexing model from the issue text.