Add a `user_new_form` hook, with context. props johnjamesjacoby, strangerstudios, jeremyfelt, DrewAPicture. fixes #18709.

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Inline documentation for hooks in wp-admin/includes/class-wp-plugin-install-list-table.php.

Props naomicbush.
Fixes #25357.

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git-svn-id: http://core.svn.wordpress.org/trunk@25547 1a063a9b-81f0-0310-95a4-ce76da25c4cd