Issue #3273317 by clayfreeman, larowlan, tim.plunkett, danflanagan8: Add 'in_preview' variable to block and layout templates

Issue #2546754 by sdstyles, joshi.rohit100: block.html.twig documents a "block" variable, which doesn't exist

Issue #2349625 by mortendk, alexpott, lauriii, Manuel Garcia, saki007ster, emma.maria, DickJohnson: Copy block templates to Classy

Issue #2226863 by cs_shadow, filijonka, joelpittet, pguillard: Update stale references to theme functions that have been converted to Twig

Issue #1972122 by akalata, mortendk, JamesLefrère, pakmanlh, ckrina, sun: Remove the DIV tag around block content.

Issue #2328913 by lanchez, lauriii, joelpittet | crowdcg: Move block classes out of preprocess and into templates.

Issue #2114563 by joelpittet, martin107, Cottser: Remove TwigReference with the help of 'without' filter replacing 'show'/'hide' functions.

Issue #1053648 by mdrummond, lokapujya, Cottser, Jeff Burnz, Les Lim, Gábor Hojtsy: Convert site elements (site name, slogan, site logo) into blocks.

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

Issue #1898034 by Cottser, jenlampton, joelpittet, Shawn DeArmond, idflood, Hydra, artofeclipse, rich.yumul, chrisjlee, gnuget, c4rl, thund3rbox: block.module - Convert PHPTemplate templates to Twig.

Issue #1927608 by EclipseGc, tim.plunkett, effulgentsia: Remove the tight coupling between Block Plugins and Block Entities.

Issue #1875260 by Gábor Hojtsy, xjm, brantwynn, EclipseGc, Bojhan: Fixed Make the block title required and allow it to be hidden.

Issue #1591806 by tim.plunkett, ryanissamson, jenlampton, Risse, psynaptic, Gábor Hojtsy, kmcculloch, pakmanlh, Rob Loach: Change block 'subject' so that it's called (admin_)label like everything else on the theme layer.

Issue #1535868 by EclipseGc, tim.plunkett, xjm, Jody Lynn, sdboyer, naxoc, tizzo, effulgentsia, dawehner, disasm, beejeebus: Convert all blocks into plugins.

Issue #1480854 by tim.plunkett, Noyz, Bojhan, yoroy, kika: Convert operation links to '#type' => 'operations'.

Issue #1760530 by jenlampton, Albert Volkman, crazyrohila, Risse, justafish, Fabianx: Move all template files provided by core modules and themes into a 'templates' directory inside that module directory.