installer: Open skin screenshot in new window

By default the plain links hijack the current window, thus
causing the user to lose their current changes to the installer
form. It may also cause confusion as to how to get back.

Absence a modal way to view the image, at least it should default
to opening the image in a new tab.

Change-Id: I4a25f1f51e04a33f1f359f12af25b8e1db5fbd20