Issue #2260061 by Jelle_S, attiks, Wim Leers, mdrummond, mgifford, holist: Responsive image module does not support sizes/picture polyfill 2.2

Broke testing UI. Reverted.

This reverts commit dc5ef576.

Issue #784626 by Wim Leers, Manuel Garcia, jcisio, joelpittet, Bevan, fcaspani, dcmouyard: Default all JS to the footer, allow asset libraries to force their JS to the header

Issue #2368797 by Wim Leers, dawehner, rteijeiro: Optimize ajaxPageState to keep Drupal 8 sites fast on high-latency networks, prevent CSS/JS aggregation from taking down sites and use HTTP GET for AJAX requests

Issue #2027623 by hampercm, larowlan, hussainweb, nick_schuch, droplet, sun, clemens.tolboom: De-fork jQuery Joyride and update to latest stable release

Issue #2254319 by quicksketch, jhedstrom: jQuery UI Spinner CSS file not added in system_library_info()

Issue #2393125 by hussainweb, droplet, mitrpaka, tarekdj: Update underscore and backbone library to latest release

Issue #2207629 by droplet, tarekdj, Devin Carlson, rpayanm: Update matchMedia library to latest release

Issue #2307717 by Wim Leers, micbar: Fixed Inform Drupal in *.libraries.yml via a new per-JS file "minified" flag whether a JS file has been minified.

Issue #2113911 by LewisNyman, Manuel Garcia, longwave, nod_, Rajendar Reddy, pakmanlh, olemedia, rteijeiro: Modal style update.

Issue #1574560 by andypost, Rajendar Reddy: Update jQuery Form Plugin to the latest in the git repository.

Issue #1921044 by pakmanlh, Manuel Garcia, rteijeiro, dasjo, aboros, sqndr, LewisNyman: Align the styling of tool tips to be more consistent with Seven.

Broke Bartik, still needs discussion.

This reverts commit 5a8a34e8.

Issue #2171071 by jessebeach, dawehner, chakrapani, longwave, Gábor Hojtsy, xjm, Wim Leers: Rename drupal_add_library() to _drupal_add_library() and remove its uses.

Issue #1996238 by sun, nod_, damiankloip, Wim Leers, longwave, alexpott, Xano, mdrummond, Mark Carver, Jeff Burnz, highrockmedia, joelpittet, et al: Replace hook_library_info() by *.libraries.yml file.