Update plugin install screens to use the new stars from Customizer/themes

[helenyhou]

For consistency, we should use the new stars from the Customizer and theme install screen in the plugin install list table and the details Thickbox. These new stars use a single sprite and are much easier to implement and maintain across screens and different colors.

I would also suggest that these same stars be used on WordPress.org Extend, since the view in the Thickbox mirrors that design closely.

20626.diff​ should take care of this in the admin and also adds the old star.png and gray-star.png to $old_files. Will need to add stars-rtl.png​ for RTL.

Related: #17428, #10148

+1 for stars-rtl.png or a graphic which can be used for both types.

Then we can remove my small hack for the theme install sidebar too, see http://core.trac.wordpress.org/browser/trunk/wp-admin/css/wp-admin-rtl.dev.css#L1642

This needs also an update because of [20730].

In [20754]:

Update plugin install screens to use the new stars. Add stars-rtl.png. Remove star.png and gray-star.png and add to old files list.

Props helenyhou
Fixes #20626