[TASK] Move close.html to ext:backend

To clean the typo3/ directory we move close.html
to ext:backend.

Resolves: #68108
Releases: master
Change-Id: Ieee52bd3ebea8f5c3f1c7f594d6d565864e07a76
Reviewed-on: http://review.typo3.org/41169


Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>