Bartik region 'Highlighted' moved from the main column to between header and featured top

The "Highlighted" region has been moved from the top of the main content, to between the header markup and the "Featured top" region.

"Highlighted" spans the full width of content, and by default contains Drupal's system "messages".

Screenshots

Of the block/region demonstration at admin/structure/block/demo/bartik

Initially

After adding "Messages", then renaming it to "Highlighted"

Note that at the time of writing the "Highlighted" region spills out of the fixed-width layout: it's wider than "Featured top". However, a fix to this is planned in #2462221: The highlighted region in Bartik is missing layout styles..

Impacts

This will impact:

• Any Bartik subthemes.
• Also configuration in admin/structure/block.
• Configuring individual blocks into regions will appear the same; however, the effect will be different (the region is in a different place on the page.)

Full history of changes

To avoid confusion for anyone encountering this CR previously, the full history of changes involved is below:

1. Initially it was hoped to minimize the number of regions in Bartik, to avoid confusing new users. For this reason, the "Highlighted" region was at first removed (#1340640: Remove "Highlighted" region from Bartik).
2. However, when the Drupal messages were turned into a block, a new region was required to present them to the user (#2289917: Convert "messages" page element into blocks).
3. This new region was also named "Messages", which was considered unsatisfactory; so the region was renamed to "Highlighted" (#2470807: Rename the default "Messages" region for all themes to "Highlighted").