Administration: Introduce extensibility to posts and comments list table...

Administration: Introduce extensibility to posts and comments list table views, for accessibility purposes.

At default, expands the excerpt view to become an extended view. Includes a new `table_view_mode` filter to allow further configuration.

Fixes #49715.
Props joedolson, audrasjb, afercia, whyisjake.


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git-svn-id: https://core.svn.wordpress.org/trunk@48167 1a063a9b-81f0-0310-95a4-ce76da25c4cd