$wgCopyrightIcon now uses $wmfHostnames['bits']

Prod and Beta were both pointing to bits.wikimedia.org, with the
introduction of $wmfHostnames['bits'] and wikimedia-button.png being in
the git repository, we can now vary the bits domain.

Change-Id: I9be2104c24639cf8688c52d70d5846916a1d3ce0