Revert "Issue #2784311 by Jmdrawneek@googlemail.com, mgifford, andrewmacpherson, xjm, Cottser, tim.plunkett, dman, DomoSapiens: Skip to main content should link to main tag or similar"

This reverts commit ed82b95e.

Issue #2784311 by Jmdrawneek@googlemail.com, mgifford, andrewmacpherson, xjm, Cottser, tim.plunkett, dman, DomoSapiens: Skip to main content should link to main tag or similar

Issue #2470807 by LewisNyman, Dom., davidhernandez, emma.maria, lauriii, jp.stacey: Rename the default "Messages" region for all themes to "Highlighted"

Revert "Issue #2470807 by LewisNyman, Dom., davidhernandez, emma.maria, jp.stacey, lauriii: Rename the default "Messages" region for all themes to "Highlighted""

This reverts commit 249e520b.

Issue #2470807 by LewisNyman, Dom., davidhernandez, emma.maria, jp.stacey, lauriii: Rename the default "Messages" region for all themes to "Highlighted"

Issue #2289917 by Wim Leers, mdrummond, lauriii, Manuel Garcia, emma.maria, Scionar, davidhernandez: Convert "messages" page element into blocks

Issue #2395825 by emma.maria, jennyOlsen, DickJohnson: Remove closing tag comments in template files

Issue #1903048 by DickJohnson, emma.maria, lauriii, mbrett5062, JamesLefrère, lilGemVinny: Revise Bartik template indentation inline with best practices

Issue #2227869 by joelpittet: Remove magic printing from Attribute class to make Twig 'without' filter work for attributes.

Revert "Issue #2218039 by sun, dawehner: Render the maintenance/install page like any other HTML page."

Broke update.php.

This reverts commit 10b03890.

Issue #2226185 by galooph, filijonka, Cottser: Remove @ingroup themeable from core theme Twig template docblocks.

Issue #2073621 by Cyberschorsch, m1n0: Remove unneeded variables from template_preprocess_maintenance_page().

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 #1938840 by chrisjlee, duellj, Cottser, shanethehat, idflood, tlattimore: Bartik.theme - Convert PHPTemplate templates to Twig.