garland 8.x-2.0-alpha1

Changes since 8.x-1.0-alpha8:

• by joachim: Fixed colours and borders in status report table.
• #2659860: Fixed clearing of title by adding a 'title_tabs' region.
• by joachim: Fixed footer region CSS rules gone missing from stylesheet.
• by joachim: Fixed admin CSS rules gone missing from stylesheet.
• by joachim: Removed stray code accidentally committed from local development branch.
• by joachim: Moved page title styling to block div for clarity.
• by joachim: Fixed vertical alignment for title & tabs.
• by joachim: Changed layout.css to be in the layout level of CSS in the library definition.
• by joachim: Removed version property from .info, as we're no longer a core extension.
• #2646334 by joachim: Updated for title, tabs, actions are now blocks.
• #2646344 by joachim: Updated for logo and slogan output in branding block.
• by joachim: Updated for core's removal of the passthrough twig filter.
• by joachim: Removed obsolete messages variable and mention of messages region in docs.
• #2465869 by joachim: Moved styling for header menus from the ULs to the regions.
• by joachim: Changed shadow at bottom of header to be made with CSS box-shadow rather than background gradient, to allow the header to change its height.
• #2465605 by joachim: Changed page layout to use flexbox; replaced background images with CSS gradients and box-shadows.