Fix links to the GNOME wiki

Update links to use wiki.g.o instead of live.g.o and point the brand
guidelines link to the right place

https://bugzilla.gnome.org/show_bug.cgi?id=742013
Use the Boxes website URL from configure.ac

Update links to the homepage to use Config.PACKAGE_URL to take the
homepage URL from configure.ac rather than writing it out by hand.

https://bugzilla.gnome.org/show_bug.cgi?id=742013