Issue #2587755 by aerozeppelin, NickWilde, swentel: AJAX error when using progress bar on file field widget

Issue #2500527 by dawehner, tim.plunkett, effulgentsia: Rewrite \Drupal\file\Controller\FileWidgetAjaxController::upload() to not rely on form cache

Issue #1954892 by dawehner, David_Rothstein, tim.plunkett: Replace 'theme callback' and hook_custom_theme() with a clean theme negotiation system.

Revert "Issue #1954892 by dawehner, tim.plunkett, David_Rothstein, effulgentsia: Replace 'theme callback' and hook_custom_theme() with a clean theme negotiation system."

This reverts commit fc04601c.

Issue #1954892 by dawehner, tim.plunkett, David_Rothstein, effulgentsia: Replace 'theme callback' and hook_custom_theme() with a clean theme negotiation system.

Issue #2051097 by dawehner, Manuel Garcia, tlattimore, neclimdul: Change 'pattern' to 'path' in *.routing.yml files.

Issue #1981368 by tim.plunkett, Crell, webchick: Convert all routes to 'module_name().foo_bar()' naming convention.

Issue #1987602 by glennpratt, juampy, pwieck, vijaycs85, dawehner, tim.plunkett, effulgentsia: Convert ajax_form_callback() to a new style controller.