Issue #2203407 by nod_, sun: Replace #attached library array values with provider-namespaced strings.

Issue #2152203 by joelpittet, InternetDevels, rteijeiro,  steveoliver, hussainweb, shanethehat, jenlampton, kpa, AnythonyR, EVIIILJ, kgoel, Cottser, dsdeiz, hanpersand: Convert theme_container() to Twig.

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.

Issue #2072647 by thedavidmeister, herom, jwilson3, Cottser, LinL: #theme 'maintenance_page' should support render arrays in #content.

Issue #2058761 by thedavidmeister: PHP notice when #attributes is not set with #theme_wrappers() 'container'.