Default Themes: Import Twenty Nineteen from the 5.0 branch.
Merges [43808,43821,43842,43860,43892,43904,43909,43926-43929,43956,43961-43963] from the 5.0 branch to trunk.
Props allancole, karmatosed, kjellr, yingling017, mrasharirfan, milana_cap, fabiankaegy, westonruter, jorbin, netweb, b-07, khleomix, audrasjb, nielslange, mmaumio, richsalvucci, littlebigthing, dimadin, joyously, anevins, peterwilsoncc, dannycooper, iCaleb, siriokun, technosiren, travel_girl, azchughtai, ianbelanger, nadim1992, ismailelkorchi, nativeinside, chetan200891, grapplerulrich, ocean90, joshfeck, frankew, AbdulWahab610, mendezcode, eliorivero, melchoyce, joen, laurelfulford, mdawaffe, kraftbj, dsmart, nao, mayukojpn, enodekciw, ketuchetan, atanasangelovdev, poena, sharaz, artisticasad, mukesh27, burhandodhy, crunnells, aprakasa, themeroots, imonly_ik, tlxo, youthkee, brentswisher, smyoon315, mrahmadawais, desideveloper, Kau-Boy, mor10, mikeyarce, dingo_bastard, xkon, twoabove.
Fixes #45424.
Built from https://develop.svn.wordpress.org/trunk@44149
git-svn-id: http://core.svn.wordpress.org/trunk@43979 1a063a9b-81f0-0310-95a4-ce76da25c4cd
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