Partially revert [20100]. The list_args JS variable cannot be extended due to...

Partially revert [20100]. The list_args JS variable cannot be extended due to its use as an argument in fetch-list. see #19815.

We should reattempt extending the args created in WP_List_Table, but will need to deprecate the current list_args to do so.

Also, infinite scroll on themes pages is no longer broken. Go team.

git-svn-id: https://core.svn.wordpress.org/trunk@20104 1a063a9b-81f0-0310-95a4-ce76da25c4cd