Fixes #21455 - Optimise admin images

I went over the PNG images with a combination of optipng and advancecomp, and the GIFs with gifsicle.

 django/contrib/admin/static/admin/img/changelist-bg.gif       | Bin 58 -> 50 bytes
 django/contrib/admin/static/admin/img/default-bg-reverse.gif  | Bin 843 -> 835 bytes
 django/contrib/admin/static/admin/img/default-bg.gif          | Bin 844 -> 836 bytes
 django/contrib/admin/static/admin/img/icon_searchbox.png      | Bin 667 -> 368 bytes
 django/contrib/admin/static/admin/img/inline-delete-8bit.png  | Bin 477 -> 395 bytes
 django/contrib/admin/static/admin/img/inline-delete.png       | Bin 781 -> 707 bytes
 django/contrib/admin/static/admin/img/inline-restore-8bit.png | Bin 447 -> 363 bytes
 django/contrib/admin/static/admin/img/inline-restore.png      | Bin 623 -> 557 bytes
 django/contrib/admin/static/admin/img/inline-splitter-bg.gif  | Bin 102 -> 94 bytes
 django/contrib/admin/static/admin/img/nav-bg-reverse.gif      | Bin 186 -> 178 bytes
 django/contrib/admin/static/admin/img/nav-bg.gif              | Bin 273 -> 265 bytes
 django/contrib/admin/static/admin/img/tooltag-add.png         | Bin 967 -> 119 bytes
 django/contrib/admin/static/admin/img/tooltag-arrowright.png  | Bin 1063 -> 200 bytes