[FEATURE] Follow up: Add autofocus attribute to BE login

Adding the HTML5 autofocus attribute to the
backend login box's username field.

Additionally adds the attribute to t3skin too.

Resolves: #49228
Releases: 6.2
Change-Id: Ie59a89ae4b4a7d490f1355ce867daa40ea7b2d54
Reviewed-on: https://review.typo3.org/21555
Reviewed-by: Markus Klein
Tested-by: Markus Klein