Add b:, q:, n: etc as interwiki prefixes for all projects
Without this change, w: works on all Wikipedias (pointing to enwiki) but
e.g. b: doesn't work on Wikibooks.
With the advent of Global User Pages (the first real foray into interwiki
transclusion, unless you count Commons file description pages), users want
a way to reliably link to various wikis in a way that will work from all
sites. Currently such interwiki links have to go via "m:" (Meta) which is a
silly hackish solution.
Ideally these would simply be local interwiki links (i.e. recursive links
that point back to the individual project), but I think we are forced to
point them to the English language versions to avoid an inconsistency to
the well-established "w:", which goes to enwiki on all Wikipedias.