Issue #2587685 by Antti J. Salminen, lauriii, jaakko, AjitS, Sagar Ramgade, rang501: Remove references to non existing preprocess functions

Issue #2575737 by Cottser, davidhernandez: Copy templates, CSS, and related assets to Stable and override core libraries' CSS

Issue #2187113 by Cottser, joelpittet: Incorrect usage of attributes in twig templates resulting in possible duplicate attributes

Issue #2407743 by mortendk, danquah, rteijeiro, davidhernandez, sivaji@knackforge.com: Remove classes from system templates s*.html.twig

Issue #2348783 by larowlan, Cottser, kugta | skein: Fixed drupal_set_message inherits previous style if set to 'status'.

Issue #1946240 by eiriksm, David_Rothstein, Yaron Tal, madhusudanmca, rszrama: Remove the hardcoded 0 index in status-messages.html.twig.

Issue #2047175 by mgifford, jessebeach: Make the status message field discoverable by assistive technology agents; alert AT agent users to error messages.

Issue #1939082 by shanethehat, larowlan, jenlampton, Cottser, LewisNyman, duellj, Mark Carver, joelpittet: Convert theme_status_messages() to Twig.