Linking attrs on guiseq and keyseq

Let guiseq and keyseq do linking:

https://github.com/projectmallard/projectmallard.org/issues/20

It turns out that that, through a fluke of implementation, linking
attributes already working on keyseq and guiseq in yelp-xsl. So it was
just a matter of updating the spec and schema, and that was trivial.

https://github.com/projectmallard/projectmallard.org/commit/48637e

--
Shaun