Bundled Themes: Make twentynineteen the default theme.

After [44149], we can now make twentynineteen the default theme. Twentyseventeen has been a great default, but 5.0 and twentynineteen is guten.

Merges [43809,43954] from the 5.0 branch to trunk.

Fixes #45152.
Props jorbin, SergeyBiryukov, pento, mcsf.


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git-svn-id: https://core.svn.wordpress.org/trunk@43981 1a063a9b-81f0-0310-95a4-ce76da25c4cd