frontend: change remove templates message

When removing templates the confirmation dialog contained message:
"Template(s)"
Changed to:
"Are you sure you want to remove the following items?"

Done this by making use of the standard RemoveConfirmationPopupView

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1001874
Change-Id: I1c6d0168a3be5f1aa0df7836167e4a78eddccf0e
Signed-off-by: Tomas Jelinek <tjelinek@redhat.com>