Pass href to linelabel on grouped_timeseries

This was broken as part of the work on refactoring how series and axes
are defined in JSON. The bug manifests itself as the <a> elements in
linelabels having no href attribute, because it's not being passed
along to the linelabel.
Merge pull request #286 from alphagov/bugfix-links-on-linelabels

Pass href to linelabel on grouped_timeseries