There are a number of minor changes in this commit :
1. Combine the "Themes" and "Components" links in the admin sidebar into
a single tab labelled "Themes and components"
2. The combined tab links to the `/admin/config/customize/themes` page
(titled as "Themes and components")
3. Add a new "Components" tab to the "Themes and components" page.
There's already an existing "Themes" tab
4. Add a "back to" link at the top of individual theme/component page to
navigate back to the respective tab in the "Themes and components" page
5. Remove the themes/components list/sidebar that currently serves for
navigating between themes/components
6. Remove the header in the theme/component page
Changes 4–6 apply only if the admin sidebar is enabled; they have no
effect otherwise.
Internal topic: t/146006.
f87e5aab0b