Various internationalization bugs in the Installer

Translator cannot reorder the arguments

config/chroot_local-includes/usr/lib/python2.7/dist-packages/tails_installer/creator.py:292: warning: 'msgid' format string with unnamed arguments cannot be properly localized:
                                                                                                      The translator cannot reorder the arguments.
                                                                                                      Please consider using a format string with named arguments,
                                                                                                      and a mapping instead of a tuple for the arguments.
config/chroot_local-includes/usr/lib/python2.7/dist-packages/tails_installer/source.py:49: warning: 'msgid' format string with unnamed arguments cannot be properly localized:
                                                                                                    The translator cannot reorder the arguments.
                                                                                                    Please consider using a format string with named arguments,
                                                                                                    and a mapping instead of a tuple for the arguments.
config/chroot_local-includes/usr/lib/python2.7/dist-packages/tails_installer/utils.py:54: warning: 'msgid' format string with unnamed arguments cannot be properly localized:
                                                                                                   The translator cannot reorder the arguments.
                                                                                                   Please consider using a format string with named arguments,
                                                                                                   and a mapping instead of a tuple for the arguments.

Unused translations due to translatable string being computed at runtime

I see lots of occurrences of the _("string %s" % variable) pattern, while it should be _("string %s") % variable, i.e. replacing the %s argument after fetching the translation of "strings %s".