[BUGFIX] Install Tool: target="_blank" on externals links

Add target="_blank" to the external links at the bottom of the About
module in the Install Tool in order to avoid problems with
X-Frame-Options of typo3.org

Resolves: #76482
Releases: master, 7.6
Change-Id: Icb7f9697769d510a641301df2a889d28f735707e
Reviewed-on: https://review.typo3.org/48474


Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Riccardo De Contardi <erredeco@gmail.com>
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: Frank Naegler <frank.naegler@typo3.org>