Issue #3196973 by casey, nod_, andypost, yogeshmpawar, droplet, Wim Leers, justafish, finnsky: Use Mutation observer for BigPipe replacements

Issue #2469431 by Wim Leers, Fabianx, swentel, nod_, yched, dawehner, tstoeckler, catch, borisson_: BigPipe for auth users: first send+render the cheap parts of the page, then the expensive parts

Issue #2580049 by Cottser, Manjit.Singh, NickWilde, alexpott, Wim Leers: Removed CSS files not removed from library definitions

Issue #2566771 by Cottser, mortendk, jstoller, LewisNyman, mdrummond, Gints Ērglis, sqndr, Chernous_dn, Manjit.Singh, axe312, emma.maria, davidhernandez, lauriii, jaxxed, akalata: [Voltron patch] Move all remaining *.theme.css to Classy

Issue #2236855 by rachel_norfolk, stefank, ngocketit, lauriii, LewisNyman, alexpott, yuki77, rteijeiro | mortendk: Use CSS for file icons in file fields.

This reverts commit 354b6647.

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.