Twenty Nineteen: Update theme description and tags.

The original theme description in Twenty Nineteen was intended as a placeholder; this update replaces it with a proper description, and removes the unnecessary `theme-options` tag. 

Props jobthomas, kjellr.
Fixes #45693.


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git-svn-id: https://core.svn.wordpress.org/trunk@44207 1a063a9b-81f0-0310-95a4-ce76da25c4cd