Move sidebar code to new "sidebar container" block

[ryelle]

Companion PR to a change in wporg-mu-plugins: WordPress/wporg-mu-plugins#397

Alternative to #58, because this also removes the aside wrapper from the sidebar.

Screenshots

There should be no visual change.

How to test the changes in this Pull Request:

1. View an article
2. The table of contents should appear in the sidebar (if the screen is wider than 1200px, or inline if less).
3. If the ToC is in the sidebar position, and it's short enough, it should be sticky to the screen as you scroll.
4. Overall, there should be no behavior or visual change from production.