Issue #2329753 by lauriii, davidhernandez, LewisNyman, nathandao: Move html classes from preprocess to templates

Issue #1982256 by mortendk, rteijeiro, tim.plunkett, LewisNyman, tlattimore, yannickoo, ry5n: Clean up html.html.twig markup.

Issue #1843650 by Mark Carver, jenlampton, Cottser: Remove the process layer (hook_process() and hook_process_HOOK).

Issue #2013094 by pwieck, Cottser: Remove unnecessary template_preprocess() reference from all Twig files

Issue #1363112 by mparker17, mgifford, tlattimore, Jacine, dcmouyard, bowersox, ry5n, ahdiaz: Simplify names of 'element-x' helper classes.

Issue #1961872 by Cottser, widukind, foopang, shanethehat, nikkubhai, c4rl, thedavidmeister: Convert html.tpl.php to Twig.